In my world I have a big region that annexed four others in just two years by using strong military strategies and propaganda. They manage to maintain this forced union using their religion, which is the most developed one and that gives religious explanation for unexplained phenomena.

Then my question is: Could it be possible for those 4 regions to accept this religion in ten years or less?

Some clarification:

  • It takes place in a medieval era.
  • The other regions have some beliefs but theirs are more primitive and disparate.
  • The other regions have less developed militaries, however they are not under-developed, they are only more pacifistic.

  • The religious education is really strong right after the annexation.

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    $\begingroup$ All religions have explanations for all unexplained phenomena; that's their job. $\endgroup$ – Erik Jun 14 '17 at 6:33
  • $\begingroup$ Welcome to WorldBuilding! If you have a moment please take the tour and visit the help center to learn more about the site. Have fun! $\endgroup$ – Secespitus Jun 14 '17 at 6:38
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    $\begingroup$ Define "accept". It's not hard to imagine that some people will accept it easily (especially the ones looking for personal/political advantages), while others might cling to the old religion(s) for many generations. Depends on many factors. $\endgroup$ – Florian Schaetz Jun 14 '17 at 6:50
  • $\begingroup$ To add to what @FlorianSchaetz said by defining accept - how much of the population do you care about and what level of acceptance? Do you need more than the aristocracy to convert? They'll have reasons to convert to stay in power with the new rulers. The merchant class too? Tell them they cannot trade with your massive empire unless they convert. The peasants? Offer incentives - free grain to all who make it through religious service. Whether any of that is real belief - who knows? But when the next generation see their parents playing along, they'll have no reason to doubt your religion. $\endgroup$ – Philbo Jun 14 '17 at 9:14
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    $\begingroup$ As this is your first question on the site here is a little tip for the future: it's generally a good idea to wait at least 24 hours before accepting an answer. This gives people in different timezones the chance to have a look at your question and at the answers, which could improve the quality of the answers. Furthermore some people might be discouraged from answering if they see you accepted something already. As of writing this your question has been open for only 5 hours and has only 252 views. In the end it's of course your decision and you can unaccept/ accept as often as you like. $\endgroup$ – Secespitus Jun 14 '17 at 12:11

11 Answers 11


The other regions have some beliefs but their are more primitive and disparate. They are more pacifists.

Those two things are contradictory; pacifism is more advanced. Nearly all animals (including humans) have a natural inclination toward violence for securing their "survival goods" (food, water, shelter, hunting territory); it is why we have been at constant war in one place or another for literally all of history. A pacifist nation has a moral philosophy against the use of violence, which your aggressive nation does not have.

That points to a cohesive (not disparate) and advanced (not primitive) religion, like Buddhism or the "turn the other cheek", "give him your coat", and "render unto Caesar what is Caesar's" form of Christianity.

I would not assume your pacifists are just cowards, afraid to fight. That is contrary to human nature, especially the behavior of young men that are more rebellious and ready to prove themselves to win status and females. (I am not being sexist; this is basic human psychology across many cultures.)

So unlike other answers here, I will be contrarian: I think your setup makes it much less likely a region will convert in the time frame allowed.

People that subscribe to pacifism so strongly they'd rather be conquered than fight are not going to ditch that for a new religion that allows violent coercion in just 10 years. Violent coercion is what they have been regarding for a lifetime as primitive, it is why we call such violent conquerers "barbaric" and "brutal" and "beastly".

How to fix it.

Get rid of the pacifism. They can be fractious tribes with poor organization and mistrust of each other; with perhaps uneasy truces between them. Each tribe can have its own local versions of religious beliefs, including the knowledge that they are the only few thousand people that believe their particular version, making it much more likely, after being militarily conquered, that they will ditch their religion and 'join the crowd', believe what the hundreds of thousands or millions believe.

As for the conquering nation; they can equate openly preaching heresy (against the State religion) with rebellion that is punishable by death. They don't have to punish private practice at all, and can even say so: Religions whither and die quickly if they cannot be publicly practiced; for a variety of reasons in human psychology, including wanting to fit in socially. If everybody is going to church for the State religion, others want to go too.

Whatever religion exists in the conquered regions to start, make it actually primitive: violent, bloody, and above all, fractured into a hundred similar but disagreeing factions in different towns and regions, even to the point of mutual hate or demonizing each other's beliefs. This will make them reluctant to put aside their differences, and thus too slow to cooperate in repelling an invader. You implement something like this, from Martin Niemöller:

First they came for the Socialists, and I did not speak out— Because I was not a Socialist.

Then they came for the Trade Unionists, and I did not speak out— Because I was not a Trade Unionist.

Then they came for the Jews, and I did not speak out— Because I was not a Jew.

Then they came for me—and there was no one left to speak for me.

It is poor writing to make things too easy; were I writing, I'd make these battles costly and bloody for the conqueror. They would win, but I don't want my reader bored, the reader needs to think the conqueror may be overreaching, and could run out of soldiers.

Pacifist religions tend to make a virtue of subjugation and non-violence, one that will be rewarded in the afterlife, while violence in general will be punished in the afterlife. So the pacifists see their aggressors as ensuring themselves a lifetime in hell, while their own pain and suffering under the heel of the oppressors is guaranteed by their Gods to be temporary, they will be with their kind in Paradise, free and living in peace for eternity. This is a belief in Karmic Justice; the bad guys get what they gave a million times over.

They won't give that belief up just because you are abusing them, that would make them the bad guys.

On the other hand, if their religion is already violent and they already believe in the principle that "Might makes Right", then your wars are more violently entertaining, and it is believable that, because Might makes Right, the conquered adopt the religion of the conquerors, because clearly the conquerors religion led them to victory and their own religion led them to defeat.

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    $\begingroup$ Not that I dislike your answer, but I wonder if the statement "pacifism is more advanced" really holds for anything but the most primitive cultures and it was also not specified that the religions themselves made them more pacifist. But this might be an entirely different question - can a country that is less developed culturally be realistically more pacifist in an low-tech setting? $\endgroup$ – Raditz_35 Jun 14 '17 at 10:57
  • $\begingroup$ Thanks a lot, your answer change the way I see the problem. I will quickly make some radical changes to make this thing more realistic. $\endgroup$ – PelleAcier Jun 14 '17 at 12:11
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    $\begingroup$ @Raditz_35 A pacifist individual may be using pacifism as a cover for cowardice (even lying to themselves about that). But that cannot be true for a pacifist nation for which pacifism plays a role in them being conquered: That requires a cohesive belief system that it is morally wrong to fight, even in self defense, or in defense of their children, or to prevent their wives and daughters from being raped. IMO that would have to be a religion and a more complex, developed philosophy than whatever the beliefs are of the smash and grab aggressors. But that would not serve the story well. $\endgroup$ – Amadeus-Reinstate-Monica Jun 14 '17 at 12:39
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    $\begingroup$ Please define what you mean by "advanced" vs "primitive." I'm not advocating for human sacrifices to appease the gods, but it feels ethnocentric to apply a blanket statement that one religion is "advanced" and another is "primitive." I agree that most main stream interpretations of the current major world religions is a peaceful one. I also agree that many religions that were main stream in the past had a more warlike stance. Perhaps you were trying to make a distinction along those lines? $\endgroup$ – Erik Jun 14 '17 at 23:07
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    $\begingroup$ @Erik Violence as a tool is pervasive throughout nature, including in natural human psychology. Early religions embrace this; many examples in the Old Testament. It takes more coherent reasoning and complexity to reject violence, despite it being everywhere, because clearly it is a cause of horrific pain, misery, grief and despair and thus (in the pacifist view) inherently evil or wrong, no matter who does it. Foregoing violence demands more rationality than a smash and grab philosophy; and just like in technology, this greater "coherent complexity" is what IMO makes it more advanced. $\endgroup$ – Amadeus-Reinstate-Monica Jun 15 '17 at 11:31

How the Romans avoided this problem

The Romans had a very practical system to work around this problem; we call it the interpretatio Romana. Basically, what they did was pretend that the religion of conquered people (and of allied people and in general of any new acquaintanted people) was "essentially" the same as the Roman religion; the alien gods were assimilated with Roman gods, and all the practical aspects of the alien religion were simply left in place as being merely local variations.

The best known example is the notional equivalence of Roman and Greek gods and goddesses; we all "know" than Minerva is the Roman name of Athena, Juno is the Roman name of Hera, Venus the Roman name of Aphrodite, Jupiter the Roman name of Zeus, Mars the Roman name of Ares and so on. This gives people a fuzzy sense that the Romans and the Greeks had the same religion, and this was the actual officious position of the Empire. (There was no "official" position. The Roman state scrupulously avoided saying anything official about any non-Roman religion.) In reality, of course, Mars and Ares, Minerva and Athena, Venus and Aphrodite are profoundly different divinities, and the purely abstract Roman religion couldn't be more different than the anthropomorphizing and luxuriant Greek religion; but politically the position was that the two religions were essentially the same and that was that. Similarily, Egyptian deities, Gallic deities, Germanic deities were given officious Roman equivalents. (For the curious, Odin is officiously the same as Mercury, Thor is just a barbarian name of Jupiter, and Freya is of course Venus; this is plainly seen even today in the names of the days of the week -- Mercuris dies is Wednesday, Jovis dies is Thursday, Veneris dies is Friday.)

So in general the religious assimilation of a newly conquered province in the Roman Empire was a no-operation: of course they had the same religion as the Romans, they just used some specific local rituals and had funny names for the gods in their barbarian tongue. The one province where this did not work was Judaea, where the locals worshipped the One True and Jealous God Yahweh; we all know where that went -- three bloody rebellions, Masada, the destruction of Jerusalem, and eventually the hostile takeover of heaven by this new provincial god.

To clarify some questions implied in the comments, this was entirely a one-directional process. The Romans saw the Greek gods Hermes and Aphrodite, or the Germanic gods Odin and Freya, as equivalents of the Roman Mercury and Venus; this does not in any way imply that they required the Greeks or the Germans to use the Roman names, or to somehow fuse the local cults with the imperial cult. Moreover, whenever a local divinity was just too alien to be assimilated with a Roman divinity they left it as they found it. For example, both the Levantine goddess Cybele and the sister and wife of the Egyptian god Osiris, She Whose Name Cannot Be Used on the Internet, enjoyed quite a bit of popularity in Rome, although they never made it into the state religion.

Further considerations

As for "maintaining this [political] union using religion" this is highly unusual. Religion is a very weak force in the political realm, and it is rarely if ever efficient at forming a basis for a political structure. No Christian Church, and not even the otherwise very cohesive Islam, has ever succeeded in transferring religious unity to the political realm. For suitably chosen values of "always" there have always been more than one Chistian political structure and more than one Islamic political structure.

A "really strong religious education" is highly specific to the Abrahamic religions -- the Hebrew religion, Christianism and Mahommedanism. The religious education in ancient Greece and Rome was purely practical and done in the family; there was no such thing as a Roman (pagan) catechism, and they would have been very surprised by the idea.

But wasn't the Catholic church powerful?

A commentator requested an expansion addressing the apparent power of the Catholic Church. The short answer is that from the 5th to the 15th century the Church was powerful or at least influential because it was a large and well structured organization which provided a decent education to its members, who were quite often called upon to serve in the political structures; its power had little to do with the professed religion.

The first observation is that the question is asking about a religion, not about a religious hierarchy. To illustrate the difference, consider that before the 19th century there was no doctrinal difference between the Catholic and Orthodox churches; they were clearly practicing the exact same religion. (Even today, they officially accept each other as fully holy and apostolic churches, and recognize the validity of the sacraments performed by priests of any of them.) But their hierarchies have been completely separate ever since the Great Schism in the 11th century.

It is the hierarchy which had power (occasionally and regionally) and influence (for longer times and over larger territories). But such hierarchies are not the norm; today they exist only in some versions of the Christian religion, such as the (Eastern) Orthodox churches, the (Western) Catholic church, and the Church of England; in the Antiquity only the Hebrew religion had such a hierarchy; the ancient religions of Greece, of Rome, of the Gauls, of the Germans, did not have anything similar.

The second observation is that the Christian religion did not do much to preserve the political unity of Europe. As in, it did nothing. Ever since the division of the Empire in the 4th century Europe has always been composed of multiple states, often at war with each other, even though they were all Christian. And eventually even the religion split into several variants, which for quite some time provided fuel to incomprehensibly cruel religious wars, until in the 17th century a silent consensus was reached placing religion firmly outside the political realm and strictly into the personal sphere. (That's why Europeans are baffled by the readiness with which American politicians speak about their religious faith. We know how dangerous this can be.)

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    $\begingroup$ Thank you, it is a really interesting answer and it gives me new ideas. However I wasn't exactly searching for a way to reconcile two different religions, but more to use the religion for cohesion. Anyway I'll think about it, thank you again ! $\endgroup$ – PelleAcier Jun 14 '17 at 12:05
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    $\begingroup$ @Jay: "Allah" means God in Arabic; it is not a name. We don't know the proper name of this particular god with 100% certainty because it is given in Biblical Hebrew and in that language only the consonants are written: "Yhwh". The best guess is Yahweh. The reading "Yehowah" is a mistake (long story). That the Hebrews, Christians and Muslims worship the same god is obvious, because those religions actually share a large body of mythology and they all agree that the god they worship is the One God of Abraham; that's why they are called collectively the Abrahamic religions. Seriously. $\endgroup$ – AlexP Jun 14 '17 at 19:47
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    $\begingroup$ @Erik: By the time of the French revolution the French state had had complete control over the French church for about 250 years; see the Concordat of Bologna (1516). The Universal Church never succeeded in uniting western Europe into a single political structure; it never even succeeded in uniting frakking Italy into a single political structure. Sure, it had influence, mostly in Italy, in Spain, in Portugal and, before the 16th century, in France. But western Europe was always a mosaic of states, quite often at war with each other. $\endgroup$ – AlexP Jun 14 '17 at 23:39
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    $\begingroup$ @alexp Jews, Christians, and Muslims all agree that the Hebrew scriptures are sacred scripture. But they most certainly do not "worship the same God" in any practical sense, as is easily proven from the fact that as we converse here Muslims in the Middle East are killing Christians for failing to worship their God. Google "christians muslims worship same god" and you'll find numerous statements by leading Christian organizations saying they do not. For example, a Catholic cardinal: freerepublic.com/focus/f-religion/3560709/posts ... $\endgroup$ – Jay Jun 15 '17 at 3:55
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    $\begingroup$ ... Many Muslims don't think so either: In Malaysia it is a crime punishable by fines and imprisonment for a Christian to refer to his God as "Allah". world.time.com/2013/10/15/… Yes, there are plenty of people out there who say that Christians and Muslims worship the same God, but most "traditionalist" leaders on both sides say they do not. $\endgroup$ – Jay Jun 15 '17 at 3:58

Doing it in ten years will depend on how converted you want them to be, how much people you want to convert and how far you're willing to go. While in ten years you can easily have a majority of people accept the new god, you can't convert all of them. Furthermore, you can't really guarantee they aren't just pretend-converted.

Fear works fast

Fear usually works well to manipulate people. Torture doesn't get reliable information because people will say anything to get it to stop. On the other hand, people will say anything to get it to stop. If your goal is to make them say your god is cool and theirs suck, don't read any further.

You could also genocide unbelievers for similar fear-striking results. It may be a fast solution, but it may also reflect badly. Particularly if your religion features "you shall not kill" as one of its primary rules. People might have a moment of brilliance, put two-and-two together and start revolting because your religion is hypocritical, so be careful with the executions.

Lost in conversion

The problem you face here is how genuine people are about your religion. Someone with survival instinct may show up for mass like a good believer, and then just sit their praying to their old gods in their head with a smile, and you wouldn't see the difference.

Conversion is hard to force. It's hard to convince someone that the thing they believed for all of their lives is wrong. Forcing change on someone may have all sorts of reactions, like resentment and hate.

Therefore, while force and fear may have immediate positive results, it could backfire later. Like they say, who lives by the sword, gets their head chopped.


I may be going to be wildly anachronistic but bear with me.

What you are asking for is change. And the oldest trick in the book of change management is: make them think it's their idea.

Consider this: what if you just put your religion in the competitive space? Build new cool churches, and hold receptions with free food or whatever will get people to come. Your religion has an explanation for anything, you may hold a Q&A sesh where every question is awarded. You know you have the answers anyways, right?

Make them compare your religion with theirs, and then make it so the comparison favors yours. They may not all come to the conclusion your religion is better, but some will. And since they'll think they came up with that, they'll believe much harder than if you just point a gun at their head.

And the bonus is that it's a double whammy. Your religion will be there, projecting a positive image. Sometimes, you just have to repeat something until it becomes the accepted universal truth. That's what advertisement is, and it works. Your religion has to be there, it has to remind you that it exists, and it has to show that it is cool.

I wouldn't go as far as advertising your religion as a new disruptive spiritual paradigm in the religious headspace, that might be a bit too avant-garde, but telling people that your god did it enough might just convince them he actually did. At worst, it won't be that foreign god that came out of nowhere, it'll be just a normal part of their life they don't reject. Non-rejection is actually a pretty good result.

Total conversion

Okay, but you still can't convince everybody, so what do you do with the rest? Nothing. They'll die off eventually. The ultimate long con isn't to change the present, it's to change the future, and future starts with the kids.

That's why the Church starts indoctrination early always welcomes children. Although I guess it could also be for the diddling, but that's another issue. Kids will believe anything: Santa Claus, God, Disney princesses, that life is fair and they'll all be equal, etc. That's why any good dictator has to be on point with education.

It's hard to convince someone that the thing they believed for all of their lives is wrong. And when that thing they believe is the thing you want them to believe, then it's a good thing.

The kids will become adult believers that will teach their kids that your god is the best. They may even convince their parents because what they learned at school must be true.

The side effect is you just created a religious dictatorship.

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Well, why not. But here is the problem:

This has happened multiple times throughout history. The Germanic and Celtic tribes, Rome, the Meso/South American civilizations just to name 5 (out of how many?). Every time they didn't just accept the new religion but basically created a new one that survived until this day.

All the above accepted Christianity - sometimes rather quickly, 10 years is certainly not too short - and they stayed that way. But with this, Christianity also accepted them. I do not want to go into details, this is definitely worth your time if you want to research this, but I want to give a couple of examples: After the Irish Celts became Christian, they stuck to some pretty barbaric rituals (barbaric by any standard). In return, they send missionaries across Europe and established countless monastic traditions that were Pagan in origin. Of course everybody also knows about the South American people that accepted catholicism but to this day are worshiping their old gods hidden in the virgin Mary and similar instances. Something like this is true for any case I am aware of. Why do people celebrate Christmas? Basically because Romans became Christians. The Merovingian Franks accepted Christianity super quickly but kept some traditions like polygamy (one example out of many) for a century or more afterwards.

Yes, you can convert your people in 10 years. This has happened with the defeated as well as the victors. But you cannot expect no feedback and no traces left of the old faith. Mostly people will follow the religion their current king is following. All this stuff you see on TV where there is always that one guy that "wants to stick to the old traditions" - well, that's TV. A very common way to do this is to marry their king to one of your religious daughters.

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There are plenty of cases of this in history. The Roman Christianity vs Paganism was a rather slow process whereas the Spanish forced conversion of Muslims was quicker but still took place over thirty years (though hindered by the Pope making the Spanish king protect freedom of religion, not a problem that seems to arise in your case).

In the end, though, each case is different. How devout were the followers before you tried to convert them? Do you approach it with violence and force people to change? (In which case you'll probably still have some underground worship). Do you try to spin it as an evolution of belief?

It all depends on the religions and how compatible they are. Without very precise description of each faith I'm afraid all we can say is that yes, it could be as quick as ten years.

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Religious education/brainwashing, when accompanied by supportive methods like exeecution/persecution of misbelievers can happen in the time span you envision.

Also a conquerer with higher civilization can indirectly support adehesion to the new religion ("if our god Whatishisname could not protect us while their god Coolgod made them so strong maybe it is better to worship the Coolgod").

Definitely possible, then.

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The simply answer is "Of course".
The more complex is "naaah that depends".
And it depends on size of the territory, the character of previous religion (if it's animism, sympathetic magic or personification of causes in humanoid form).

For example Poland have switch to Christianity in 966. Well, the prince and his court have been baptised. Some regions didn't heard of this until XIV century. And their pagan rituals and belief just morphed into polytheism. And just 70 years later Bezprym said "You know, this Christ mumbo jumbo is bollocks, let's not go there, it's a silly place". And he send king crown to Emperor Conrad II so he was cool with it.

Oh, and also in 1410 Poland had this big war about being real Christians with the real Christians that were real because they killed pagans.

So in ten years some parts of the conquered regions could not even heard that they are conquered.

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Looking at it historically, I'm hard pressed to think of examples where a conquered people have accepted the conqueror's religion like you describe.

I can think of x cases:

  1. Sometimes people of one religion manage to convert others through missionary activity. Christians have been very successful at this, converting people all over the world to Christianity. Sometimes this was done after or as part of military conquest, but quite often not. Buddhists and Muslims have also done this successfully.

  2. Sometimes people hold on to their religion despite incredible pressure from the conquerors. The Jews are probably the best example of this. They were conquered and their nation was destroyed and they were scattered across the world, and there was no such thing as a "Jewish country" for almost 2000 years. Yet Jews kept their religion.

  3. The conqueror might exterminate the local people and replace them with their own people. This is pretty much how Muslims converted north Africa and the Middle East. But no one is really converted here, the people who believe in the old religion are simply wiped out and their land taken over by people with the new religion.

  4. As AlexP discusses, the Romans came up with the interesting -- and highly successful -- plan of merging their religion with that of the conquered people. They'd say, "Oh, you have a god of the sea that you call Poseidon? Why, we worship him too. We call him Neptune." This worked at least partly because the religions were similar enough that making these analogies might have been a stretch but it did not sound totally insane. This didn't work with the Jews because the idea of many gods and the idea of one God were just too far apart. You can say, "We have a god of thunder, a god of the sea, and a god of war. You have a god of thunder, a god of war, and a god of the harvest. So okay, let's put them together and then we can both have 4 gods." It doesn't work to say, "We have 100 gods and you have 1 god, so let's put them together and now we have 101 gods."

And frankly, I think a big part of making it work was that the Romans did not care very much about religious doctrine, so they were willing to be flexible. The Romans could shift from Jupiter as a philandering husband who would turn his girlfriends into animals or trees to hide them from his wife, to Jupiter as an icon of perfect justice, without missing a beat. Christians and Muslims have resisted saying that Jehovah and Allah are just two names for the same God because they both have very detailed, specific ideas about their Gods that are incompatible, and neither is willing to just toss them out.

  1. For any religion, there are going to be people who are very serious about it, who build their lives around this religion, and there are going to be people who don't care much, who will check the box for this religion on a form because that's what their parents were or that's what most people in their community are. If someone is nominally a member of religion X but barely knows what this religion teaches, goes to whatever worship services they have on major holidays once or twice a year, etc, it seems plausible that he could be easily converted to be a nominal member of religion Y. Especially if there is some benefit to converting, ranging from "if you want to get one of these high-paying government jobs, you have to convert" to "if you want to avoid being tortured and killed".
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  • $\begingroup$ Good example of how succesful the Roman strategy was! Jupiter the Roman god was not a philandering husband. That was Zeus the Greek god. The Roman religion did not even have a body of epic myths as such -- their gods were mostly abstract ideas. (Lady Justice, Iustitia, is the last Roman goddess new statues of whom are still made in the 21st century.) The Romans accepted the Greek myths as nice literary tropes, suitable for poetry and such, but they were never part of the state religion. $\endgroup$ – AlexP Jun 14 '17 at 19:57
  • $\begingroup$ "Hard pressed to think of examples where a conquered people have accepted the conqueror's religion": when the Arabs took Syria, the Levant and Egypt from the Easter Roman / Byzantine empire in the 7th century most people were Christians; one hundred years later most people were Mohammedans. Conversely, Spain, Portugal, Malta and Sicily are now Christian; the (re-)conversion from Mohammedanism was equally swift. $\endgroup$ – AlexP Jun 14 '17 at 20:02
  • $\begingroup$ @AlexP I don't think it's true that no one is making new statues of other Roman gods. For example, designtoscano.com/product/…. If by "new" you mean not just, a statue made today, but, not a replica of an ancient statue, maybe that's true, but one would have to go through many examples. Not like that's the main point, just an amusing side note. $\endgroup$ – Jay Jun 15 '17 at 3:27

That depends entirely on what you understand under conversion:

  • Paper conversion: If conversion means that the converted speaks some creed, becomes a member of some religious organization, and takes part at their rituals, you can do this pretty much within a single year. Just go round to everyone, and force them to either accept your conditions, or face the consequences. True, you might have a few subjects less after that, but the others are all "converted".

  • True conversion: If conversion means changing their inner allegiance, you cannot do this within the lifetime of a human being. No matter how much force you use, the people who are really devoted to their own religion won't change their hearts and minds. You may force paper conversion on them, but you can bet that a good part of their traditional religion stays around.

    Given a strict "education" (= brainwashing every child within state-organized, obligatory schools), you can hope to somewhat eliminate the old religions over the course of a single generation, but definitely no quicker.

There is only one loophole to this: If you can somehow "prove" that your religion is a super-set of their old religions, that they can continue pretty much in their ways as long as they also honor your gods, you can make conversion sufficiently easy to allow for true conversion to happen within ten years. This works really well if your religions are compatible, sharing the basic concepts, and not ruling out any of the fundamental concepts of the other religion. However, if there are any fundamental discrepancies between the religions, this may not work at all.

This is pretty much what the romans did: You were free to worship your local gods, as long as you also accepted and worshiped the roman emperor as a god. This worked quite nicely for most other religions of the time, except for those stubborn jews and christians who would not accept any god beside their own. You see, the jewish/christian claim that there is only one true god stood in fundamental opposition to the roman polytheism, and it stopped those jews/christians from accepting the roman emperor as their god. They would even turn down paper conversion, and choose to be executed instead. The romans entirely failed with converting those.

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Likely not.

For the atheists of the today, religion is (simplified) some extraordinary ritual and the reading of gibberish books. For most religious people today, the religion is (simplified) the knowledge of the ancients and also a phylosophical viewpoint. In a medieval era,

  • science doesn't exist. Note the difference: it doesn't mean that there is a science but it is too primitive. The terminology didn't exist. Religion fills its role.
  • moral doesn't exist, religious laws fills its role. Of course everybody knew, for example, that killing somebody is a very bad deed, but how is it bad and why is it bad, there were religious answers.
  • a basic knowledge of the world doesn't exist, religion fills its role. Planets, stars, sky, earth, thunder were all religious terminologies with religious explanation.
  • the religion was also the state-organising ideology. The reason of the power of the King wasn't like "this is the law" (as today), rather that "God gave him the power". Note, this is not a God of some ancient book, but a God who really does exist, and if he wish, can kill us all or make us happy forever. In the mind of the medieval people, the hierarchical structure of the society didn't end on the Earth. The hierarchy lasted until the heaven, until God, and the local power (king) was only the procurator in the visible world.

The result is that in a medieval people were much more religious as the religious people of the today. They lived in a world, where God is not only a philosophical terminology, rather God, angels really do exist.

The result is that changing a religion - really - is unlikely in a single generation.

This was also a reason, why the Romans simply inserted the gods of the conquered people into their, politheism. It even happened with montheistic religions, despite that the same "injection" was impossible. Simply elevating the role of saints, holidays similar to the conquered people, made their conversion more likely.

There is another significant difference. In a medieval world, although the criminal law can be hard (and not always codified), the practical power of the rulers is far from the today. Personal identification didn't exist. If somebody committed a crime (for example, committed partisan attack against the conquerors), he could have flee to a swampy or uninhibited region, or simply far away. If nobody knew him, he left unpunished. There were no photos. There were no taxes. There were (mainly) no census.

There is a historical example. After the collapse of the (West-)Roman Empire, next to the social collapse, the Christianity conquered Europe. The Barbarian tribes, founded states with pagan religions, often these states were converted to the Christianity. A timeline, in the Hungarian history:

  • the pagan Grand Prince converted to the christianity roughly at A.D. 970-980. Although he baptised, he still followed also the ancient shamanistic religion.
  • his son were crowned to a king in 1001. He was the first Christian king of the country.
  • He also imported a foreigner ruling class (priests, knights and so on) to strengthen his power.
  • There were many blood, uprisings, burning the pagan scripts, killing anybody won't convert, and so on.
  • The last major pagan uprising happened in 1046.
  • The last out-of-society pagan groups died out roughly a century later.

In this case, the conversion of the country lasted roughly 70 years long. It is a 3 generation switch.

Another very important thing: a religion shift means a social shift, too. For example, a chaotical warlord-based state should be reorganized as a Christian kingdom. Don't focus exclusively on the religion, focus to the social processes happening with it (and by it) concurrently.

Thus, not only a religion change happens, rather

  • Society, government reorganization (in the example: free fighters of the pagan state were lowered down to peasants in the christian kingdom)
  • Power changes. In the era it wasn't common to lose power and survive it. In the example, a priest/knight class was imported by the Christian king to help to kill win his local opponents (note, he was also a traitor in the eye of the pagans, by calling foreigners against his own people).
  • Technological changes (f.e. pagans have bows and arrows, it is unusuable against the plate mails of the christian knights, castles were built)
  • And, the religion shift happens with these all concurrently, it is not alone.
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Here's an interesting element you could try out that might make the 10 year goal more feasible.

I think one reason why Christianity spread so quickly in the first century was because a major supernatural event happened which changed the world forever (of course, in the perspective of the new Christians) - that event being the recent death and resurrection of Jesus of which the apostles were first-hand witnesses. It was also seen as the fulfillment of many prophecies.

In your world, there could be a major event caused by the god(s) (or at least attributed to the god(s)) in recent history that makes the religion seem more credible and urgent. If people think the religion's gods are real and powerful and active, they'll be more likely to convert, I imagine.

And I don't know, maybe throw in some prophecies for good measure. Everybody likes a good prophecy :).

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