Was Socrates an atheist? Yes, according to the persecution:

Socrates: Or, do you mean to say that I am an atheist simply, and a teacher of atheism?

Meletus: I mean the latter - that you are a complete atheist.

The hemlock followed. After which, Plato learned to be discrete. He just wrote the dialogues and it is his characters that have doubts. Like Euthyphro who has a dilemma and Meno that learns that virtue is given, by God, to the virtuous. Plato has plausible deniability. Plato himself is not the one who says the things that are spoken by the stranger from Elea:

Socrates: Is he not rather a god, Theodorus, who comes to us in the disguise of a stranger? For Homer says that all the gods, and especially the god of strangers, are companions of the meek and just, and visit the good and evil among men. And may not your companion be one of those higher powers, a cross-examining deity, who has come to spy out our weakness in argument, and to cross-examine us?

Theodorus: Nay, Socrates, he is not one of the disputatious sort-he is too good for that. And, in my opinion, he is not a god at all; but divine he certainly is, for this is a title which I should give to all philosophers.

The divine anthropomorphized as lovers of wisdom.  ¶ 10:45 AM