The Bible, including the Old Testament, had not yet been assembled when Plato and Aristotle were writing.

No, they weren't all bound up in a book and branded "The Bible" or "The Old Testament". But the laws of God did exist (It's very possible that Adam wrote some of the Old Testament), and have been existent as long as man has. Josiah found the 'lost book of the Law' in 2 Kings 22. It wasn't called 'the old testament', but it was still the word of God.
It doesn't matter whether Plato ever read what we call the Bible or not. What matters is that he presupposed concepts of absolutes, of laws of logic, of absolute right and wrong, et cetera, when those things can't be justified without a Christian worldview. Because in any other worldview, (it's hard to generalize because it's a different argument with different types of worldviews) there is always an inconsistency with good and evil being defined. But with Christianity there isn't.

.....we are still awaiting any sort of evidence that it wasn't from any of the other contacts the Greeks had or - perish the thought - something they developed themselves? But I suppose you would declare that to be really christian as well. Oddly enough, you seem happy to call everyone as essentially Christian who says the aren't, except for those people who think they are ...

Okay, so taking your scenario, what if they *did* borrow the concepts of Christianity from some other venue. Where did they get it? You can't escape the question just by suggesting that it came from some other source.....because we still have to ask, then where did it originally come from? Anyway, where is the evidence that they did get it from 'some of the other contacts'? What other religion gives a basis for good and evil?
Good and evil dates back to the garden of Eden, when God first told Adam and Eve not to touch the tree of the Knowledge of Good and Evil. They didn't obey. And since all men are descended of Adam and Eve, of course all men have a sense of good and evil.

"you seem happy to call everyone as essentially Christian who says [they] aren't, except for those people who think they are......"
*chuckle* no, I'm just pointing out that most Western culture atheists must borrow from a Christian worldview in order to reason. But that does not make them 'essentially Christian'. Just because I borrow some sugar from the neighbor in order to make candy doesn't mean that I am essentially 'neighbor', if you follow me.