A long time ago, I read something by Plato (translated). Throughout most of it, they were planning a festival and wanted Socrates to come up with a tale of Greece's heroic past.
Socrates' part was mostly about Atlantis.
But one of the other two characters gave a treatise on the universe's creation and described his belief (proxy for Plato's own views?) that there was a supreme
entity who had set things in motion. He believed everything on earth was an exact (albeit tainted due to mortality) copy of its counterpart in heaven. He also reasoned that a doorway needed to be created to perfect man. Only a half mortal-half immortal being could facilitate such a portal.
Does anyone know what play I am talking about?

This is not a trick question. I really have forgotten. As I said it's been years since I read it and would like to pick it up again, this time paying more attention to the Atlantis subject.