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:
- Try to convert pagans who don't believe in your religion
- 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?