[I]f you say there is no God and then turn around and tell me I should not be a racist ... and I say, “why should I?” how do you respond? If we are all evolutionary accidents, why can’t I believe and practice anything I wish? Perhaps you respond that there are laws prohibiting discrimination. To that I answer, “Suppose the laws are changed, is it then OK to discriminate?” It was once legal to own slaves, but did that law make slavery moral?
Thomas's query begs a question in response: Is belief in God the only reason you're not a racist? To put the question a little differently: Is belief in God the only reason not to be a racist? To think so, as Thomas apparently does, presumes that man is absolutely incapable of recognizing all human beings as fellow human beings and treating them the way he'd want to be treated. As usual, religion presumes that people are hopelessly stupid and incapable of civilization without guidance from a god. But if religion distrusts human intelligence, it distrusts human will even more. It's human will, after all, that explains why theoretical Thomas shouldn't be a racist and can't practice anything he wishes. Civilization is an expression of human will, but human nature will always distrust human will to some extent, both on the "who are you to say?" level of nihilistic individualism and Thomas's "how do I know you won't change your mind?" level of theological skepticism, which sees all secular civilization as nihilistic individualism. The answer to such skepticism is that the will to civilization, founded on reasoned principles, creates a standard of accountability to which anyone who changes his mind or betrays his ideals, even the original lawgiver, can be held. Civilization depends on will but isn't founded on will. It depends on the will to uphold and enforce the principles reason has discovered (and continues to discover) against both individualist and theological nihilism -- the belief that one's own will vetoes the will of the whole world, on one hand, and the belief that civilization is founded on nothing but a will in which reason has no voice and which sees reason as an enemy.
To return to Thomas's original question: if we reason that people should not be treated differently due to race and will that races not be treated differently, then we will that Thomas shouldn't be a racist whether he can reason his way to that point or not. And if someone changes the law to permit racial discrimination without refuting the original reasoned objections, the will to civilization should work for repeal -- by legal means first and then however reason and will deem necessary. God is not necessary for reason to be compelling to man, but will is necessary for reason to amount to anything. Theological skeptics often get reason and will mixed up because their universe is founded on a will upon which, so they believe, reason depends. Those who know better recognize that reason is independent of will as well as its judge, and that no one can undo reason by an act of will. Those who try have a god to sell, or sell themselves as gods. They're not as different as people like Thomas might think.