Effectively all large societies have been religious. But religions haven’t been successful because they’re true: practically all religions are mutually contradictory, so there must be another reason to explain their success. One very good reason—probably the most fundamental one—is that religions have been very useful in promoting social cooperation. This is the basic prerequisite for civil society and economic progress.
Securing cooperation is the fundamental challenge in establishing a civil society. It would be nice if we could all just get along, but we all have incentives to cheat and steal on occasion. These incentives become magnified as the society grows and relations become more anonymous, since it becomes more and more difficult to know whether other people are trustworthy. There are various ways to solve this problem, but if left unchecked these incentives would lead to social chaos.
Religion is a particularly effective, if crude, method of securing cooperation. In general, religions lay out moral rules (which often happen to be social ones) and set up strong incentives to follow them. Eternal life in heaven is an infinitely great reward for being a cooperator; eternal damnation in hell is an infinitely great punishment for being a cheater; and the judge is omniscient, so it’s impossible to “get away” with anything. Societies with these religions would have a competitive advantage: greater cooperation means more trade and more production—in a word, prosperity. These societies would grow and spread—by conquest or consent—until they came to dominate. So religion is a hack that gets people to behave in large, anonymous societies.
On the individual level, there are strong incentives to portray oneself as a believer. Being genuinely religious makes you more trustworthy, as you can be counted on to cooperate and not cheat. Displaying (advertising) your religiosity to others is a signal of this trustworthiness, it creates a good reputation. Thus, being religious has material incentives: more people to trade and cooperate with. Once a religion gains a foothold, the incentive would be for everyone to jump aboard.
Yet I don’t think that religion could get a foothold with these incentives alone. Religion—a cultural universal—fundamentally rests on our psychological willingness to believe in the supernatural. Evolutionary psychology plays a large role. So religion is a particularly infectious meme that exploits an innate human irrationality and produces the byproduct of social cooperation, creating strong incentives to be religious.
This explains so much about religious behavior. Why do the religious often ask what keeps atheists from stealing and murdering? Because that’s supposed to be the function of religion. (Note that this question is self-contradictory: on one hand, it tries to argue that only religion can be the source of morality, while on the other hand, it presupposes that theft and murder are inherently wrong—regardless of what god says.) Why are the religious so hostile towards atheists, and why are they less hostile to believers in contradicting gods? Because genuine belief in any type of divine justice makes one more trustworthy; atheism makes one an unconstrained danger. Why do people invest so much time and money in religious affairs? To signal to others that they’re believers and therefore trustworthy. Why is questioning religion taboo? To avoid undermining the social order.
But there’s plenty of hope for truth. As other institutions replace religion in enabling social cooperation, religion becomes redundant and people can freely satisfy their intellects without undermining the social order. Religion is supported by mass belief and cultural momentum. It has already started to come undone, and will only unravel faster as mass support shrinks.