For Dennett, to prove the biological origin of belief in God is to show its irrationality, to break its spell. But of course it is a necessary part of the argument that all possible human beliefs, including belief in evolution, must be explicable in precisely the same way; or else why single out religion for this treatment? Either we test ideas according to arguments in their favor, independent of their origins, thus making the argument from evolution irrelevant, or all possible beliefs come under the same suspicion of being only evolutionary adaptations—and thus biologically contingent rather than true or false. We find ourselves facing a version of the paradox of the Cretan liar: all beliefs, including this one, are the products of evolution, and all beliefs that are products of evolution cannot be known to be true.
(Not surprisingly, Dave Lull also sent me this link.)