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Human beings have colonized a planet in the outer habitable zone of a K-type star with approximately 1.5 Earth oceans of water, living in peaceful coexistence with mixotrophic birds who saved the first colonizers from a deadly duocellular microbe. Now, a new religion (which I will call the join) has emerged among the humans, a religion with two tenets that will affect my question:

  1. Try to convert pagans who don't believe in your religion
  2. After a sufficient period of time (maybe 1/2 year) kill the pagans who didn't convert to your religion.

For my fun and entertainment, this faith is generally spread on a somewhat-central island in a midsize island archepelago with population 40 million. These people are smart and will band together to accomplish their goals. No nukes, but everything else is on the table. The birds, who are sentient and whose values contradict with the religion, cannot be converted. I want to make a believable fight in which the join defeats the rest of the humans on the archipelago just barely. How large of a population would have to be in the join?

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  • $\begingroup$ Why too broad? Please elaborate. $\endgroup$ – JSCoder says Reinstate Monica Mar 29 '18 at 18:20
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    $\begingroup$ You mean, in % of population? Does not need to be very high, only need to control the government while this government is not a democratic one. Everyone will convert. $\endgroup$ – Alexander Mar 29 '18 at 18:37
  • $\begingroup$ They don't need that many people. An invader can rule a much larger nation. Think of the Mongols $\endgroup$ – Vincent Mar 29 '18 at 19:16
  • $\begingroup$ The standard rule of thumb is that invaders need a 3-1 advantage. $\endgroup$ – RonJohn Mar 29 '18 at 19:18
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    $\begingroup$ A religion with just those two tenets is a strong indication that you know very little about religion and why it motivates people. $\endgroup$ – RonJohn Mar 29 '18 at 19:24
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The answer is "it depends. Your tenants will provide two possibilities:

  • Your believers repeatedly demonstrate great miracles, never once failing. They convert the entire population in a few weeks. Maybe 1% of the population will be enough.
  • Your believers try for half a year, and then try to kill everyone.

So unless your believers get lucky and/or do indeed have the one true religion, they're going to engage in warfare. That's all there is to it. Religions do not get supplanted in 1/2 a year without force. It takes generations.

As such, the number required is a question of warfare. To discuss that, we'll have to look at the psychology of the people, technological preparedness, etc. That's well beyond scope here.

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