I often run into progressives and relativists who insist (rightly) that "slavery is bad, torture is wrong and racism is repugnant". By the grace of God, Christians know these behaviors to be objectively wrong because God reveals this to us in his word and (secondarily) puts it in our conscience since we were created in the image of God. I would encourage you to see such a strong value declaration as an opportunity to ask postmodern skeptics some questions about their own presuppositions.
Such a declaration is odd coming from an atheist or relativist for how do they know this to be true? On what basis do they believe this? What standard are they appealing to? The most frequent answer I have been given by these kind of skeptics is essentially that human morals are a social construct. Since they affirm (absolutely) that there are no absolutes I am frequently told that their view of values/principles emerge from lessons widely drawn from human experience and around which consensus has emerged. That human rights as a language and as a normative construct came out of the horror of WWII. Such ideas emerge through consensus-building and eventually take on axiomatic existence for most people.
On what basis does consensus determine what is true? Since it is quite arbitrary of you to have determined that values and morality come forth from some kind of consensus, how do you know this method of deriving morality is right? What authority are you appealing to or presupposing that makes it so? If you cannot account for or justify it how do you know this is what everyone should do or how we should determine truth? This consensus-building concept is an appeal to something that is not self-validating. Adding more numbers to your interpretive community does not make it so. Therefore, your view is not any more unbiased (non-religious) than my own. When you make your position into public policy you are likewise promoting your dogma derived from a source you have arbitrarily determined to be authoritative. You have affirmations and denials as to what is good and bad, do you not?
You said "Slavery IS bad. Torture IS wrong. Racism IS repugnant." If you really believed that morality was relative then such statements would be unintelligible. Can you justify such concepts rationally with your worldview? Is it mere consensus telling you this? To be consistent as a relativist you could only say these things are bad for yourself. How is it not arbitrary to universally adopt such an ethic? Why does consensus make it any more valid than an individual or any other religious view? Either you are claiming it is true that these things are bad, or you are merely telling me your preference derived from a self-appointed authoritative source. And if it is merely your preference then you have no right to impose your personal ethic on society. This is an appeal to absolutes or you would not have such strong feelings toward it.
Based on your worldview:
"First, you seem to be saying that slavery wasn’t wrong until there was a consensus that it was wrong. Or that torture wasn’t wrong until we came to a consensus that it was wrong. Do you really want to say that--that slavery and torture wasn’t wrong in 1750, because then the consensus was that both were OK? If you fall back on saying that slavery was wrong in 1750 even though most people didn’t feel that way -- then you do believe in absolutes, I think."Second, what if you saw the consensus about slavery and torture eroding? What if you saw that half the world was moving toward a new consensus that slavery and torture were OK in many circumstances? (There are a surprising number of people who do think torture is OK if it might stop a nuclear attack, etc. It could easily happen.) On what basis, then, could you argue that the emerging new consensus is wrong, since, in your view, something is only wrong if there is a consensus that it is wrong? It seems that the only way you could say “reverse the new consensus” would be if you grant that torture is wrong even if the consensus changes.
Third, this is an elitist argument, because the fact is that there are plenty of cultures and places in the world that don’t agree with your ‘consensus.’ You are saying, then, that the part of the world that believes in human rights is the enlightened, correct part. When you say these beliefs take on axiomatic existence for ‘most people’ you mean ‘most people I know, the ones who are thinking properly.
Fourth, if you don’t believe in absolutes, you can only offer at best a pragmatic argument against these evils. If you were living in 1750 and you came to believe slavery was wrong when few others did, you could not argue from consensus. You would have to argue that slavery is impractical for us, that it makes for a society in which we are all unhappy. You could only appeal to people’s self-interest. Only if you agree that there are moral absolutes could you say that “Slavery is wrong regardless of whether you feel it benefits you and society or not. It is simply wrong to treat people that way. Period.” (Tim Keller)
Even without God's word, the fallen conscience of man still understands from both reason and emotion that there is objective right and wrong and they act on it daily even though it is inconsistent with their world view, for is we are but carbon units and chemical reactions then genocide is no different than a day with the kids at Disneyland. This is NOT to say that atheists cannot be moral. They may be more moral in some areas than I am. The issue is not whether they are moral (for God has implanted the seed of religion in all). Rather the question is on what basis does morality exist.