The answer to this can be found in Chapter 14. Basically, Diamond is saying that religion becomes more important as society becomes more complex and has a higher population density. In such societies (which come about because of agriculture), there are many unrelated people living together who could easily come into conflict and even kill one another. Therefore, there needs to be a government to keep the peace. But the government becomes what Diamond calls a "kleptocracy," taking resources from the people. There needs to be a way to get the people to accept this and organized religion is one way. Organized religion, then, develops as a way to justify having the sort of strong and kleptocratic government that is necessary in a complex, agricultural society.