Most ethical systems in our contemporary world can probably be subsumed under the names of either Aristotle or Nietzsche. Aristotle thought that human beings had a telos. There was something that man was for, a purpose or an end, for which he was on the earth. Virtuous acts were those which help men achieve their telos. The good life was a life which conformed to the cardinal virtues - prudence, temperance, fortitude, justice - which were objectively right to live by.
Nietzsche, on the other hand, denied that there was any overarching purpose to being human and thus there was no objective moral right or wrong. Morality was all a matter of perspective. It's a matter of how we see things, a matter of individual subjective preference. Thus the ubermensch or overman creates his own values. He rejects the "slave moralities" of theism and embraces the "master morality" of the Promethean man. This is what makes men great, and great men define their own good.
Neither Aristotle nor Nietzsche believed in the existence of a personal moral law-giver which fact makes for an odd state of affairs. Aristotle's telos makes no sense unless the purpose or end of mankind is somehow conferred upon man by a transcendent moral authority. Otherwise, where would such a purpose come from? But if there's no personal law-giver or telos-giver then neither humanity nor individual men have any purpose, and the "virtues" are just arbitrary conventions.
Nietzsche is right that in the absence of a transcendent, personal law-giver what constitutes a virtue is just a subjective choice. On Nietzsche's subjectivism the virtues extolled by the Nazis are no more wrong nor right than those embraced by St. Francis of Assisi. They're just different.
If theism is correct, however, if there actually is a God who creates man and endows him with a telos then the moral law and the classical virtues, really are objective and obligatory.
So, the way the theist sees it, Aristotle, by denying a transcendent, personal God, was inconsistent but nevertheless right about there being objective moral duties, and the atheist Nietzsche was consistent but wrong in his denial of objective moral right and wrong.
