The professor began by saying that right and wrong do not exist, they are merely cultural conventions, words societies use to describe what they approve and disapprove of. Take for example the Eskimo tribes in which the elderly are set on a block of ice and sent out into the ocean to starve to death or the primitive practice of cannibalism or the Indian practice of Sati in which many widows were burned alive on the funeral pyres of their deceased husbands. These are examples of how different cultures view right and wrong. For these people, these practices are good and right. It is their way, and who are we to speak against them?
If right and wrong really are merely subjective cultural conventions and really are relative from society to society as the naturalist (atheist) wordview tells us, then how could we ever say that one society was morally superior to another? We can not say that Hitler was wrong when he murdered six million Jews. All we can say is that we disagree with Hitler's views and that we don't like what he did.
Regardless of what some people say, we all believe in an objective law of morality. Even those who say that moral laws don't exist or morality is just relative from culture to culture actually show, by the way they think, speak and live that they really do believe in an objective moral law.
Even the atheist ethics professor believes in an objective law of morality. He immediately becomes a strict proponent of moral absolutes the moment soneone cuts in line in front of him at the checkout counter. "This isn't right! I was before you!" It will do no good to say to him at that moment, "Hey, morals differ from culture to culture, and who's to judge?"
The atheistic naturalist world view is not able to account for objective morality and our moral experiences as human beings. By contrast, the Christian theistic world view does provide a basis for objective morality.
The atheist knows that right and wrong are objectively real and he can't help his natural response to even a minor violation of the moral law. In his heart of hearts he knows it exists. God has inscribed an innate sense of justice on his very soul. The atheist can't erase is simply be telling himself over and over again that right and wrong are merely subjective social conventions. What ever he says be believes as an atheist, deep down, he cannot help but know the truth, even if he refusess to admit it to himself or anyone else.