In my world there is only one religion that has existed as far back as written history. There are many subcategories of this religion (Like Christianity has Catholicism and other subcategories) but no completely other religion. How might I explain this?
This is easy.
The God of the single religion exists and makes himself known.
Right now, we don't have scientific evidence of any extra-physical entity. And by scientific, I mean such that we could make a hypothesis, perform an experiment, and make observations that fall in line with the original hypothesis.
In your world, there is. Perhaps on the full moon, he descends to collect his offerings from his shrines. It's predictable and observable. It has an actual effect on the world. Hell, he could even have an Ask Me Anything session every once in a while to give guidance to the people.
Now, there may be sects of the religion (similar to the different branches of Christianity) but pretty much everyone would agree that there is this one religion to base yourself off of. Bonus points if he smites the occasional non-believer and takes an active role to prevent fragmentation.
This is pretty much the goal end-state of any monotheism. Say that the Islamic conquests didn't stop at Tours and the walls of Constantinople. All people submitted to Allah from England to Japan.
You may have noticed how excited ISIS is to destroy books that aren't the Koran. So now the entire world is Muslim, and all the books that aren't the Koran or written since the Koran are burned. All you have left is a mythical history of the founding of your religion, some weird artifacts from an earlier time (whats the deal with these Pyramids?!?!). How would anyone know there was ever another religion?
Due to some negative feedback, let me explain a bit further. The spread of any monotheism does not have to be correlated with a dark age and book burning, nor do I suggest that it would be. I simply state that it is possible
The choice of Islam was simply that it was the specific religion that seemed the closest (historically) to encompassing the whole world. Muslims as a historical group over the last 1400 years are no more likely to burn books than any other group, for reasons religious or non-religious.
This post is a chain of events that explains why people would think that there is only one religion. This directly answers the question, "How might I explain this".
Everyone is agreed on the existence of A God or The Gods.
For the sake of simplicity, this answer is written under the assumption that your setting has a Christianity-like religious group. The difference between sects of this religion instead originates from interpretation of doctrine, much like the variants of Christianity on Earth. There are several possible explanations for the lack of other religious groups like Buddhism, Taoism, etc.
Option One: As stated in the comments, the world may simply be so well-connected that there's no room for other religions to form without deriving from your "base" religion.
Option Two: As kingledion said, there may have been other religions in the past, which warred against each other and now only one remains, having wiped out all trace of its opposition.
Option Three: If this is a fantasy setting, the God of your religion may have acted to enforce his will upon the world in relatively recent times, perhaps as recent as a few centuries ago. In this case, there is only one religion because everyone knows it to be true, and it's more like believing in a king than a god.
In my world there is only one religion that has existed as far back as written history. There are many subcategories of this religion (Like christainity has catholism and other subcategories) but no completely other religion.
There are dual founder effects and inertia.
One is intellectual. One you make foundational assumptions in your worldview and spread that meme, it is very hard for people to think outside of that box. And, ideas that build on widely shared assumption that only innovate sparingly in key respects further cement the foundational assumptions because even in the face of differences of opinion on theology, the core beliefs become consensus views of all faiths. The developed the consensus worldview is, the harder it is for religion specialists who have to learn it all and derived status from having done so to toss it all way and start over from scratch, and non-specialists have less of a vested interest in doing so.
People innovate based on examples of past innovation, and if there has never been more than one religion to chose from than people are much less likely to come up with the idea of founding a new religion.
One is political/sociological. One a religion is up and running in the first chiefdom-ship or the first city-state, that polity is going to have an edge in being the first to expand with a complete cultural system already in place over societies that have to innovate and develop an equally fit alternative from scratch. And, even if someone does create an alternative, the first invented will be more mature and have more kinks worked out and more ancient, so it will have an edge in the battle for converts.
Once a religious worldview is in place, inertia becomes a powerful force. Absent strong intervention, people continue in the habits and customs of mind and practice that they have always had.
There are compatibility issues.
Why did everyone start following the religion of Microsoft, rather than arguably better alternatives?
Particularly in areas like explanations of a divine world that isn't experience first hand (even more so than software which is more functional in nature), your society's fitness isn't harmed materially because you choose Brand A over Brand B. But, sharing a Brand with your neighbor (whatever that may be) may enhance your ability to avoid war with your neighbor and ally together against third parties. The more people are on your side, the more valuable your brand of religion becomes and the less valuable alternative brands become relative to it and people will willingly convert to the most valuable brand.
For most of history, decisions about religion have been made for an entire people by the highest available sovereign leader. So, in a world with millions of people but only scores of countries, the task of conversion may involved scores of people, rather than millions of individual decisions. The notion of an individual autonomous right to decide your own religious beliefs is a decidedly modern one. Even as late as the Reformation, just five centuries ago, this was the rule. The Protestant Reformation was fought one ruler at time, not one person at a time, for the most part. The notion of an individual right to decide your religion if you aren't a king or a prince, doesn't really emerge until the Enlightenment in the 18th century (arguably, this is a natural intellectual consequence of democracy which transfers sovereignty to the common man, at least symbolically).
Also, culture isn't as a general rule, severable. If a new more effective culture conquerers an old less functional one, the new one is going to convert people to its religion along with all other aspects of its culture much of the time.
Who writes history?
Once written histories exist, oral history keeping tends to collapse. So, whoever writes the first written history controls the remembered past and can simply cast any part of prior religions that are hard to erase from the collective memory as mythology, stories and folktales, all of which are told in the frame of fiction rather than legendary history. The worldview of the first history writers is frozen in time and ideas omitted fade away, erased from the popular consciousness in a few generations.
Certainly, an important factor in the success of the religions that persist today and the failure of those that do not is that every single one of the several successful religions were fleshed out in written accounts, while many of those that are gone now were not recounted in writing.
Also, since literacy generally starts limited to a narrow group of people learning as adults, and is only democratized much later, the collective action problems of building a religious consensus is reduced because only a small group of people who learned to read and write after going through the same training in the same way, and having the same status in society and daily experiences as each other, need to be convinced, not a more heterogeneous group of people. Moreover, since all literate people early on are going to be accountable to the king or prince of the land, this narrow group of literate folks will also have strong cues from their leader to assist them in developing a consensus and will have an interest in presenting a unified front in order to collectively wield power vis-a-vis their leader who needs the support of at least some of them to administer his domain.
One More Point: Germ load
I'd forgotten one other interesting historical correlation. Religious diversity is strongly correlated with the amount of infectious disease/parasite load in the vicinity. This is because religions frequently have different food and purity taboos. In places with high disease load, the benefits to society as a whole of having religious diversity with varied food and purity taboos in making sure that everyone isn't wiped out by some virulent contagion because not everyone participates in the vectors that are vulnerable to it due to taboos outweigh the societal benefits of religious consensus. In contrast, the lower the infectious disease/parasite load is somewhere, the less religious diversity there is because the benefits of large amounts of consensus outweigh taboo diversity. So, you are much more likely to get religious consensus in a world with a low disease load (e.g. a place with a climate like Scandinavia) than you are in a place with a high disease load (e.g. a tropical rain forest).
Obviously, these patterns emerge through natural selection, rather than by conscious design.
In addition to the options outlined by Passage, I would propose the following:
Option 4: The religion is exceptionally inclusive. The core principles of the religion may either be open to a wide variety of interpretations or so universal that few, if any, religions would be excluded. Alternatively, the original teachings have been fragmented or obscured to the point that it isn't clear what the core principles originally were.
With this option, all of the world's religious organizations can and do identify with the same religion. (Perhaps there is a social or political incentive to do so.)
For inspiration, study Hinduism. Instead of being split into clear divisions, it's a big, interrelated family of various practices. Some Hindus follow the traditional polytheistic route and pay homage to the entire pantheon, while others will dedicate themselves to a specific deity. Some Hindus view the deities as distinct entities while others view them as aspects of the same single supreme being. Some prefer a devotional approach (i.e. prayer and rituals) while others will focus on a more introspective approach (i.e. meditation).
In your world, a similarly flexible and diverse religion may have spread early enough that it has either incorporated or eliminated all others.
Option 5: Religion is or was nearly extinct. The world religion is the only survivor.
What could possible have caused religions to disappear? There's a number of possible reasons, though how plausible they are depends upon the era in which you want this single religion established.
In a more modern setting, it could be easily explained by the growing popularity of science and secular humanism. As generations pass, people abandon religion. However, unlike our world, there is only one religion with enough organization and cultural clout to persevere.
In an ancient or medieval setting, this might be explained by the expansion of an Empire that was extremely anti-religious. (Perhaps the Emperors felt worship of deities threatened their power.) This led to a purge that destroyed most religions or forced them underground where they eventually faded away.
Option 6: A well timed catastrophe could explain the existence of a single world religion.
Imagine a religion with a prophecy of doom that involves fire from the sky followed by a long, dark winter. Now imagine that a major meteor crashes on the planet and triggers a major extinction event. Lastly, imagine that those familiar with this religion are among the few survivors. Not only will the religion spread widely as the world is repopulated, but its legitimacy will be cemented for millennia.
Different religions have typically come into being in different corners of the world. The logical solution is for your population not to be too widespread, or break down into isolated pockets which do not communicate with one another.
A second factor to consider is a military arm of the church which stomps out any serious dissenters to their doctrine. If they're aggressive and thorough enough they could subjugate other populations and simply erase their own beliefs from history (while difficult, a savage enough regime could accomplish this).
A less aggressive solution is to have simply undeniable miracles being performed on a regular basis. If your priests are capable of demonstrating their God's power on (reasonable) demand then people would convert to your religion and never question it.
For a religion to spread we need long-distance travel. Horses, boats, etc.
History begins when writing was invented in a form that survive for ages, and it gets used to describe events as they happen.
In our world, we moved from one to the other very quickly, and no religion had time to conquer the world.
In your world, there were more time. Maybe traveling was easier, maybe nobody thought writing was cool. Since this is pre-history, nobody really knows.
One way or another, there was time for one cultural group and its religion to spread all over the planet and suppress all others.
In our world, humanity hung around in Africa for a long time before spreading across the world.
What if humanity had started in a smaller, more isolated place, like New Zealand?
For a long time there would be only New Zealand. After a while, there would only be one surviving culture/religion there and this could happen before writing was invented.
Later on better boats lets people spread all over the globe.
In a sense this is a variant of the first answer, since travel is easier when all distances are small.
There will be variants on this religion. Some variants will strongly claim that this makes them different religions. E.g. "The people over there follow a god that allows them to eat pork. The One True God does not allow its followers to eat pork, so they must have a different, false, religion."
Maybe confront the problem by asking "how are religions the same" and not "how are they different". If you deconstruct religious beliefs far enough they all have the same basic content.
Christianity may have various sects which ascribe to slightly variations in beliefs. Abrahamic religions also have their various sects: Christianity, Islam, and Judaism.
Explaining the existence of one religion is easier than explaining the existence of more religions: if in your world, simply, a God actually really exists, and this God made himself evident enough to population.. why should there even be more religions at all? All would just follow that God, and that's it.
If in your world God does not exist (or a God exists, but it's not the one that your population believes in), then it can still be explained in many ways: maybe all people spread from a same area, and started desperding around the globe after creating the religion, maybe one "nation" (or tribe, or whatever) conquered all the others, maybe some preachers have been particularly successful and converted everyone all around the world (it's what most monotheistic religions aim at doing anyway)
In the history of our species on this world, religion had the role of allowing early groups of humans to cohere in groups larger than known kin and use reference to a shared belief to establish the trust required not to kill each other.
As these groups cohered in response to outsider groups, all religion had the effect of discriminating between 'ours' and 'others' religion, and therefore developed in opposition to others.
So there must be many religions.
I would say, then, that in your world, your people would, in their early origins, not to have needed to have formed these groups protecting against each other. Perhaps they are not strong enough to do each other physical damage so other people are not a threat. Perhaps resources are plentiful, but they had to evolve alongside a much more powerful dominant species who would kill them on sight, so all that mattered was escaping from these predators and not worrying about other people. Then when the predators got wiped out (by a virus or whatever) then your people were left alone with their beliefs.
How about - because the first peoples were brainwashed/inspired to believe in a single religion? See "Off Armageddon Reef" by David Weber for an example.
In this series, set in the far future, humanity exists on one world. Their technology is kept suppressed by a repressive "religion" created by the mission director a thousand years before specifically for that purpose. The first generation of colonists had their memories tampered with to make them "believers". Because the "archangels" of the religion actually did definitively exist with (brainwashed) witnesses to attest to their miracles (displays of high tech) the religion became firmly entrenched with no possibility of an alternative emerging.
Most religions are born just because they allowed some restricted group of people to have politic power, sad to say, but just see the movie "Agorà" to have an example of that.
The existence of only one religion could be explained if one of the contending parties have won and conquered the whole world. Anyhow, most people have the tendency to "manipulate" others (me too in this post indirectly), so it is hard to have 1 religion, there will be always someone that have "a different view". Unless of course your religion has very strict rules (like killing people not following it), in that case if someone or very little groups have different world views they would be very isolated.
But that is typical of humans, if your "group" of people is not human maybe they are more willingly to follow only one religion.
Another explaination could be:
- They were visited by aliens in past times, and that conditioned their culture (like what you see at the beginnin of Start Trek Movie Episode II)
- You do not need aliens, maybe some really catastrophic phenomena happened and everyone saw that (a big comet?)
You can take the history of the world we live in as an example for how it might work. Only in your world, the zealots of the most aggressive cult have been successful.
Note the deliberate wording of "aggressive cult" because it is not limited to religion, and not at all necessary that the winner is the "best fit" or even "correct".
It starts with the Neandertals, which we traditionally consider as primitive, and which we have (short of a few genes in our DNA) practically eradicated from history. There is some good evidence that Homo neanderthalensis was quite intelligent and had a quite developed culture and clear signs of higher mental abilities than any other homo at that time.
Only just, they were less concerned with breeding and expanding, and less aggressive than Homo sapiens. Worded differently, while one group was looking at the stars and trying to figure out why someone had put all those dots into the sky, the other group was busy throwing javelins at the males and raping the females.
Then there is Chrisitanity, of course. Everybody knows Jesus of Nazareth was born at Christmas, and resurrected at Easter. This truth, like most of the inalterable truth in the Bible (including the testimony of an eye witness who demonstrably lived a hundred years after Jesus of Nazareth), was decided upon at the Council of Nicea in AD 325.
Historic evidence suggests Jesus of Nazareth was not at all born in December (nor did he die/resurrect at Easter), however there happened to be the pagan festival Yule (which is how Christmas is still called in some northern countries today) which was replaced by Christmas. It is a funny coincidence that Easter sounds almost like Eostre (whose other name is Ostara, which incidentially sounds almost like Ostern in German), and by coincidence Easter is exactly at the time when Eostre used to be. All Hallows (nowadays better known as Halloween) is near, but few people remember that it's really just Samhain, another pagan feast. Thanksgiving? Lughnasad. Lughna... Lughna...what? Who wants a sacred day that nobody can pronounce on 1st October, if there is a better one on the first Sunday of October, which is a doubly-sacred day (because Sunday is already the day of the Lord). Who is this Lugh anyway.
(n.b.: USA and Canada have slightly different dates nowadays)
It also happens that many names of the days of the week are derived from ancient pagan deities but changed ever so little as to mask their origin to the casual observer. You wouldn't think Friday (which sounds like "free day" in German, and pretty much everybody thinks it refers to the weekend that follows) honors Freya, would you. You would think Tuesday might be the Two-eth-day of the week, but it really is Tyr's day. -- Who is Tyr? Tyr is dead, babe.
The stragegy behind this is simple, and effective: Rather than kill everybody worshipping the wrong gods, replace their rites with alternative, more shiny rites, and presents on the same day. Change the names a little so they're no longer recognizable. Nobody will object. After a few generations, most will have forgotten.
The story of Carolus Magnus (Charlemagne) and Widukind is another historical textbook example of how to drive forward a religion or culture, and how to completely anihilate a competing religion. Charlemagne is well-known as the splendid king who was such a good Christian and did so much good. A true shining star in history with no blemish.
Reality, however, looks a bit different: Invade, kill all who resist, hunt them down to the last man, woman, and child. Deliver Widukind, or we will decapitate another 200 innocent men. By the way, abjure your heathen gods and kiss the cross, or we will burn down your villages and kill every child and every woman. That was Charlemagne. Sounds familiar? Not much different from some present-time IS videos on youtube: Anyone who will not say "Allah is great", we will kill (I forgot the name of the guy who said that, some 25 year old "soldier" with a big beard, I think TV news said he meanwhile died in Syria).
Crazy fanatic or King and Emperor Blessed by God? The difference lies only in success. Break treaties, betray the Langobards? Oh come on, what's a treaty between friends, er... enemies. You thought breaking treaties was an US invention? Well, you're wrong.
I'm not even going into much detail about crown princes suddenly deceasing and his heirs disappearing while in custody, and their wife being held hostage, or about raiding allied cities (Pamplona, to name) on the way home from Cordoba. Everybody knows that the Basques are the bad guys for assaulting the glorious Franks. The Song of Roland clearly proves what a splendid paladins Roland and Charlemagne were, and the treacherous Basques attacked them for no reason. Let's not be too pendantic about the sixth commandment either. This is for the ordinary people. For the Holy Emperor and Protector of the Church, it's perfectly adequate to break it twice per week, or more often if necessary.
Then of course, there's the strategy of simply erasing memories. People die, and people forget, but what's written down remains. The Library of Alexandria contained a lot of texts which were not opportune with the Roman idea of truth. Incidentially, it burned down halfway in 48 BC while the Romans were around, and burned down again 300 years later, incidentially when the Romans were around again. But the Romans weren't the only ones burning books. The Catholic Church burned inopportune books and the people writing them during the entire middle ages, and even during the 20th century this was common practice (Hitler and Stalin, to name only two). Burning books has become somewhat futile due to the existance of digital media nowadays, but some circles still do it, mostly for symbolic reasons.
In summary, it is not alltogether impossible that there may be only a single religion in the world, and no supernatural or exceptional things are truly necessary for that. All it takes is the zealots of the most aggressive religion being successful.
One SMBC provides an alternative explanation. After the singularity, the one company that produces the hardware running all the minds has a patch. Afterwards, brand loyalty is at amazingly high levels.
Note that at this level of control, you don't even have (to be allowed) to notice that there are other hardware brands. So there is only one religion and it is Cloud Corp.
If the populace believes there is one true god and one true path of religion and all non-believers are the enemy etc. to the point of lynching them in the street (think back to the Witch Trials) then almost no-one would want to deviate from the one true religion.
And if your world has modern technology it would be almost impossible to hide if you did. Regular purges could be a part of life.
Anything that amplifies this would help too: rewards for informing, regular checks, church attendance monitoring, requirement to fulfil religious duties on a regular basis, even mind-reading devices hypnosis etc.
That combined with the fact that anyone overhearing even a whisper of another religion would cause them to immediately report it and send the entire village/city/town/government into a manhunt and public trial.
Think North Korea. Not necessarily religion through fear, either - religion through the absolute through-and-through belief that there is no other choice. Think of wartime when we villify the enemy (like how the Japanese were painted as literally being monsters) to the point that people adore conformity and are quick to ostracise.
You have to define your terms. You mean only one ORGANIZED religion? You imply that, say, Buddhism and Christianity aren't different "subcategories" of the "same" religion. According to who? All human mental constructs, especially ones which are held by a large fraction of humanity, will have many similarities. Getting a bunch of "authorities" to agree which "qualifiy" (and which don't) for inclusion into a religion will result in arbitrary, subjective, and inconsistent determinations. Anyone familiar with the theological debates inside the Vatican surely should understand that no two people believe exactly the same thing. The only way to have one religion is some form of direct mind control, although I could make a plausible case for having thought police with the ability to determine if someone is lying...
The God Emperor of Mankind provides answers to this (and to all questions).
It was he who took a righteous purge of all heretics upon The Earth. Going so far as to stamp out all religions and mysticism. Once this was completed he turned his mighty gave to the stars and begun to clean the worlds of man one by one. Freeing each of the tyranny and lies of religion; brining instead the light of knowledge and reason. Thus was The Imperium of Man formed and He rained on a high over His dominion.
When corruption struck and His fallen sons attempted to kill him they were defeated. After this his divinity was recognised and The Imperial Cult formally formed. This (being the only true religion) is the only accepted religious practise throughout the imperium of man. Even those of the Cult Mechanicus realised their Omnissiah was The Emperor.
TLDR: Stamp out all other religions and give your society a few hundred (or thousand years) of social dominance. History is what people read and think. Given a few dozen generations that can be anything you want it to be. (There may be some small deviation creeping in from time to time)
Pay no attention to those cultists near that large worm hole. Do not worry about those things out of the window while in warp space either, just a trick of the light. Genestealer Cults? Oh well its just some simple minds getting over-awed by a large animal. Like people find cats pleasing only with giant bug monsters. Stop arguing or it will be reported to The Inquisition (not that they exist, I just meant further up the elclisastiary really).