Moral metastasis...

In What We Can't Not Know, J. Budziszewski spends a chapter on the question of whether we could know what we owe to our neighbor even without knowing what that we owe anything to God. He writes:

...when Psalm 14 remarks "The fool says in his heart 'There is no God'," it doesn't call him a fool for thinking it, but for saying it even though yet deeper in his mind he knows it isn't true. From this point of view, the reason it is so difficult to argue with an atheist - as I know, having been one - is that he is not being honest with himself. He knows that there is a God; he only tells himself that he doesn't. You don't have to take this from a theist like me. Consider the remarks of Harvard population biologist Richard Lewontin - an atheist who thinks matter is all there is - in the New York Review of Books:

Our willingness to accept scientific claims that are against common sense is the key to an understanding of the real struggle between science and the supernatural. We take the side of science in spite of the patent absurdity of some of its constructs, in spite of its failure to fulfill many of its extravagant promises of health and life, in spite of the tolerance of the scientific community for unsubstantiated just-so stories, because we have a prior commitment, a commitment to materialism. It is not that the methods and institutions of science somehow compel us to accept a material explanation of the phenomenal world but, on the contrary, that we are forced by our a priori adherence to material causes to create an apparatus of investigation and a set of concepts that produce material explanations, no matter how counterintuitive, no matter how mystifying to the uninitiated. Moreover, that materialism is absolute, for we cannot allow a divine foot in the door.

Lewontin is admitting that he and those who think like him are only selective skeptics. They are hostile to belief in God because of an a priori commitment to a dogmatism which excludes God - a dogmatism about which they are not skeptical at all, which they accept not because of the evidence but in spite of it, and to which they will cling even when it forces them into absurdities.

What drives someone like Lewontin to fanatically follow his dogma in spite of reason? Having no religion, in the secular sense, are we to conclude that his stance is based on explanations garnered from the natural world by gathering evidence? If not... why?