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, that there was something that man was for, a purpose or an end, for which he was on the earth. Virtuous acts for Aristotle were those which helped men achieve their telos. The good life was a life which conformed to the cardinal virtues - prudence, temperance, fortitude, justice - which Aristotle argued were objectively right to live by.
Nietzsche, on the other hand, denied that there was any overarching purpose to being human and therefore 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.
To wit, Aristotle's telos makes no sense unless the purpose or end of mankind is somehow conferred upon man by a transcendent moral authority. Otherwise, how does man come to be endowed with a purpose? Where does 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" that Aristotle touts are just arbitrary conventions.
Nietzsche, on the other hand, is correct that in the absence of a transcendent, personal law-giver what constitutes a virtue is just a subjective bias. 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 true, 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.
Only if theism is true can one to be both consistent and right about the existence of objective moral virtues and duties.
