Religion as the Secret Handshake

I’ve spent a lot of time trying to reason out the existence of religion. I know it has nothing to do with truth, yet it is such a complex and universal behaviour that I have to believe it has some evolutionary purpose. First I’m going to explain why I believe it has to have a purpose, then I’ll go through a couple of theories I have as to what this purpose might be.
Why religion needs a reason:
Human social interaction is a complex thing, and it is regulated at it’s base by the way in which our brains work, this is in turn determined by our genes, whose only determining factor is survival: genes which cause their host to reproduce more get copied more themselves. Small quirks (like, say, being slightly more aggressive in certain situations) can be attributed to random mutation or even the densities of various kinds of genes, these can and will happen at random and can’t automatically be considered to be a product of natural selection. Anything complex, which describes virtually every aspect of our social behaviour, must be a product of evolution, because while random chance can change something small, you cannot build something complicated (like a human brain) by just tossing things together at random. It would be like taking a watermelon, running it through a blender, and then pouring it our into a watermelon shaped hole. How many times would you have to do that before it reassembled itself by random chance? Complex things do not occur in evolution by random chance, they can only occur when one random mutation is useful, so it spreads throughout the population, and then later there is another also random but also useful mutation builds on whatever the first mutation did, and so on. So anything complex built by evolution must be useful for survival, it must create an advantage for the organisms who have it over the ones that do not. Religion is a complex behaviour; there is evidence of it in every human culture, therefore, it must have some evolutionary use.
I used to think this could be explained purely in terms of memes. Memes are ideas, the environment they live in is our brains, and they are as tailored by evolution as our genes. I concluded that religion was an especially virulent and parasitic species of meme. The religion memes defended themselves by making any disagreement into heresy, they promoted themselves through tools like infant indoctrination and forced conversion. I believed that religion didn’t need to be useful in any way to humans, because it was not designed by genes but by memes, for whom their own survival is paramount and the survival of their human hosts only meaningful inasmuch as that effects the memes ability to survive and transmit.
Now I’m not so sure that the tendency can be entirely explained away by parasitic memes. It seems too pervasive even in an environment where there are ample counter-memes (reason), as well as massive competition from other religious memes. It seems possible, even likely, that the tendency towards religion is not just a result of their effectiveness as parasites on our gene-designed brains, but also because our brains are partially designed to be susceptible to them. Why? What use could there be in having brains that are especially susceptible to a parasite? There can be only one reason: there must be some advantage to being religious for the genes.
It seems counterintuitive. Religion limits the ability of people to apprehend reality. It takes time and energy, and creates conflict unnecessarily. Often people will live celibate lives as a result of religion- surely this dead end for genes must be selected against by evolution? Or not. Religion has all the characteristics needed to be a secret handshake. It isn’t predictable, it is complicated and people know all kinds of strange details. If you come from another group, you have no ways of knowing what the particular aspects of a religion will be. It guarantees the fidelity of people within a group, because only the people who have been with a group for a very long time can know all the important details of it’s religion. This is counterproductive in the modern world, but in a primitive world where there was no established law, and the only way to protect yourself was to be successfully defeat all competing tribes, the ability to immediately identify members of other tribes is a useful ability. The negative emotional response to people of other religions is based on this: members of other tribes should be mistrusted, because they do not have the same loyalties. The fact that this tendency is both useless and dangerous in present day doesn’t really matter, because all of human history has been an eye blink for evolution- only things that last fore millions of years have any impact on the flow of evolution. We have a tendency towards religion because it is useful in the primative environment that modern humans and all of our immediate predecessors lived in.