According to Diamond, religion evolves to be more of a support to the government as a society becomes more complex. That is, religions become institutionalized and more formalized and they are used to bolster the power of the secular government in a complex society.
In Chapter 14, Diamond argues that complex societies are ruled by "kleptocracies" that take resources from the people and must somehow keep the people happy as they are being robbed. This is where religion comes in. In complex societies, Diamond says, it evolves to be a formal religion where the priests help to justify the leaders' power.
Diamond claims that the "combination of government and religion has...functioned, along with germs, writing, and technology, as one of the four sets of proximate agents leading to history's broadest pattern" (267). In other words, the development of organized religions accompanied the formation of governments, and this was a major factor leading to the different patterns of development followed by different people around the world. The reason is that all governments engage in extracting the value of labor from the people they rule over, or represent. Diamond argues that the difference between a state that uses the resources of its people to provide valuable services to them and a state whose elites simply consume the resources of the people to finance their own lavish lifestyles is a difference of degree rather than of kind. In short, all states are to some extent kleptocracies, and all states are fundamentally inegalitarian, in that they are founded upon inequalities of resources and power. He says that elites have historically justified and propped up their status by a variety of means including force and redistribution of resources. But one of the most powerful and enduring strategies was to "construct an ideology or religion justifying kleptocracy" (277). Religion itself creates a bureaucracy of priests that are instrumental in extracting wealth from people, and they provide what Diamond calls "ideological justification" to the state. It also provides a group of people a shared ideology that helps them get along, and "gives them a motive, other than genetic self-interest, for sacrificing their lives on behalf of others" (278). Each of these is crucial to creating what a historian named Benedict Anderson called the "imagined communities" necessary to the formation of a complex state-level society.