Atheist have no moral high ground, because according to Atheism, such a thing doesn't exist.

According to Atheism, reality has no absolute morality, morals are simply human constructs that can change by society or the individual. Yet Atheist will rant and rave about the "evils" of religion as if its a fact and not relative. An example question, was slavery always wrong, or only wrong when the majority deemed it so?