Theists often ask "If my religion isn't true, then why do so many people believe in it?" First of all it should be pointed out that arguing whether or not something is true based on how many people believe it to be true is a logical fallacy. That being said, there is a phenomenon that should be explained: why do so many people believe in a religion that is not true? Note that for any religion, there are more people in the world who believe it to be incorrect than believe it to be true.

The answer to this question was provided by Richard Dawkins, an expert on evolutionary biology. The key elements of evolution are that there is something which replicates itself (genes) and there is some selection mechanism (natural selection). Dawkins argues that a similar phenomena occurs in religion: People are indoctrinated into religion at a young age. When they have children, the children are indoctrinated into the same religion. So like genes, religion replicates itself. Also like genes, the replication is not perfect: the children don't necessarily believe exactly what their parents believe. So if a selection mechanism is available, religion will evolve. Religions with good psychological appeal or religions that inspire zealots to murder the followers of other religions will be preferentially selected. Dawkins refers to this mental analogue of a gene as a "meme". Dawkins says this about religion memes:

"The survival value of the god meme in the meme pool results from its great psychological appeal. It provides a superficially plausible answer to deep and troubling questions about existence. It suggests that injustices in this world may be rectified in the next. The ‘everlasting arms’ hold out a cushion against our own inadequacies which, like a doctor's placebo, is none the less effective for being imaginary. These are some of the reasons why the idea of God is copied so readily by successive generations of individual brains. God exists, if only in the form of a meme with high survival value, or infective power, in the environment provided by human culture."

For more information on memes, see Viruses of the Mind.