Here's a hypothetical dialogue between an agnostic and a theist.
A: Evolutionary theory (whether genetic or mimetic or a combination) can explain why you believe in theism. This explanation has nothing to do with the truth of theism. Hence, you should not be a theist.
B: The explanation has much to do with theism. For a good creator would want us to believe in him, and hence, if our beliefs arose through evolutionary means (which you grant!), he would have likely set up the evolutionary pressures in such wise that they should favor belief in theism.
A: You're begging the question by depending on the theism that my argument puts into question.
B: Are you a sceptic about our empirical, logical and mathematical knowledge?
A: I would be really stupid to be such, since scepticism about any of these areas would undercut my belief in evolutionary theory.
B: Good. But now consider this claim: One can give an evolutionary explanation (genetic or mimetic or a combination) of your empirical, logical and mathematical belief.
A: Yes, I can, and I see where you're heading. There is, however, a crucial asymmetry between the theistic case and the empirical, logical and mathematical ones. The explanation of why we hold these empirical, logical and mathematical beliefs depends on the truth of these beliefs. It is useful to believe tigers are dangerous because they are dangerous and the environment is such that true beliefs about dangerous things are useful.
B: But it is useful to believe that God exists because God has set up an environment in which having theistic beliefs is useful for the dissemination of genes or memes.