Socrates, Plato, Aristotle, and others taught—and wrote—monumental tomes on Ethics, Virtue, and the nature of Good and Evil three to four centuries BEFORE Christ. Before there WAS a Christian religion!
It was before Christ came to earth, but it was not before there was a Christian religion. The Christians before Christ were not called Christians, they were called Jews. They believed in a Christ to come. They worshiped the God of Abraham, Isaac and Jacob, the God of Noah and Adam, etc. etc.
And again---like I've said before, the reason why Plato and Aristotle, Socrates and other philosophers had a basis to believe that there even was such a thing as good and evil, was because they borrowed the Christian/Old Testament presupposition that there was such a thing as good and evil. On the atheistic basis you have no foundation to believe in transcendental good and evil.