One of the standard moves in the apologetic argument from morality is the invocation of Hitler. If there is no objective moral standard, then how do we say that the Holocaust was wrong? Don’t we all feel instinctively that the Shoah was evil? Doesn’t that point to an moral standard outside of our own moral reasoning?
As NonStampCollector points out, most apologists end up refuting their own argument in an interesting way:
(via Bob Cargill)