We usually think that religions require people to believe in certain things. Like God, heaven, life after death. But what if religiosity is more akin to a tune you just can't get out of your head than a consciously arrived-at system of beliefs?
Memes, according to Wikipedia, are "ideas, symbols, or practices that can be transmitted from one mind to another through writing, speech, gestures, rituals, or other imitable phenomena with a mimicked theme."
They propagate and evolve much like genes do: through natural selection. Here's how Wikipedia says memes operate in the area of religion.
Aaron Lynch attributed the robustness of religious memes in human culture to the fact that such memes incorporate multiple modes of meme transmission. Religious memes pass down the generations from parent to child and across a single generation through the meme-exchange of proselytism. Most people will hold the religion taught them by their parents throughout their life. Many religions feature adversarial elements, punishing apostasy, for instance, or demonizing infidels.
In Thought Contagion Lynch identifies the memes of transmission in Christianity as especially powerful in scope.
Believers view the conversion of non-believers both as a religious duty and as an act of altruism. The promise of heaven to believers and threat of hell to non-believers provide a strong incentive for members to retain their belief. Lynch asserts that belief in the Crucifixion of Jesus in Christianity amplifies each of its other replication advantages through the indebtedness believers have to their Savior for sacrifice on the cross. The image of the crucifixion recurs in religious sacraments, and the proliferation of symbols of the cross in homes and churches potently reinforces the wide array of Christian memes.
Makes sense to me.
The meme hypothesis explains why core religious beliefs almost always have a strong appeal to people. By contrast, what I like to call "real reality" has lots of nasty aspects to it.
Death. Pain. Disease. Suffering. We are born. We die. In-between life and death there's both pleasure and pain, happiness and sadness, good and bad.
Such is the state of human affairs without the consolations of religion. Life after death. God's eternal love. Heavenly bliss.
It's easy to imagine how popular these notions would have been to the first Homo sapiens individuals who came up with them. And then how easily such religious memes would have found their way into other human minds. Today, get-rich-quick schemes spread like wildfire. Religious ideas would have been equally tempting to early members of our species, if not more so.
Memes don't have to be rationally (or irrationally) accepted. They're akin to viruses in that a meme can "infect" a mind without the person choosing to invite it in. People don't consciously choose to get a cold. A cold virus simply causes them to have cold symptoms.
Likewise, the meme theory of religiosity says that while religious beliefs may appear to be adopted, actually they may simply spread by virtue of their attractiveness to human minds.
Here's a short Richard Dawkins video where he talks about memes and religion.