Although many modern scholars are quick to say Plato was not a monotheist, Plato's Timaeus seems to speak of a "supreme God."

http://www.fullbooks.com/Timaeus2.html

The interpretation of many Plato scholars seems to be that even though Plato's notion of God might be unacceptable to Christians looking for the notion of a Christian God, nonetheless Plato's "supreme God" was not just a new mythology for Zeus.

So it seems likely to me that in the centuries between the writing of Plato's dialogues and the dawn of the Christian era, many people in the Greek-speaking world must have philosophized about Plato's "supreme God." However I have not found many textbooks that address the topic.

I know that the Orphic and Eleusinian mysteries had many followers. Is there any evidence that Socrates or Plato were initiated into those mystery religions?