If naturalistic evolution is true, then our cognitive faculties have resulted from blind mechanisms like natural selection, working on sources of genetic variation such as random genetic mutation. And the ultimate purpose or function or chore of our cognitive faculties, if they have one, is survival. Survival of the individual, the species, the gene, or the genotype.
Consequently, it's unlikely our cognitive faculties produce beliefs that are true. They can only help survive. Consequently, given naturalistic evolution, the probability or our faculties' being reliable is low, even when applied to naturalistic evolution itself.
--From Alvin Plantinga
"An Evolutionary Argument Against Naturalism"