Now it may be argued that Darwin's hostility to Christianity is beside the point. Shouldn't a scientific theory be judged on its own merits, rather than on the motives and psychology of its progenitor? Yes, of course – if the theory is truly scientific and confirmed by empirical observation. Isaac newton was as strange as they come; as John Maynard Keynes pointed out, Newton's private philosophical notebooks make one think of an ancient Babylonian magician. But Newton's scientific theories were rigorously formulated. They can be tested and shown to be true for most of material reality. But an ideology dressed up as a science is a different matter.
Theories like Darwinism, Marxism, and Freudianism have an explanation for everything (natural selection, economic repression, the unconscious) and so finally explain very little; they are elastic and vague enough to absorb almost any contradiction; when they run into falsifying data, they simply mutate. And since these theories began, consciously or not, as highly skewed readings of the available evidence, the biographies of their founders are very much to the point.