I recently read that along with all the Gnostic gospels which were uncovered in 1945 at Nag Hammadi, they also found a copy of Plato’s Republic. So far I’ve not seen anybody explain why though. The most information I seem to be able to get about it is a line off Wikipedia that says, about the Republic: “The original is not gnostic, but the Nag Hammadi library version is heavily modified and therefore gnostic.”

In any event, I’ve never read the Republic but I have at least a good guess as to why it’s included among these other codices. And that is, Plato’s allegory of the cave which appears in that book. As a metaphor, it’s more or less exactly what gnosticism is all about: that this world we experience is an illusion, or incomplete. That what we perceive as prisoners in the cave is nothing more than the shadows cast on the wall by that which is truly real. Plato talks about how the goal of the philosopher (literally “lover of wisdom;” ie, the gnostic) is to free himself from the cave, and go out into the world of true forms outside the cave.

Wikipedia’s entry on the cave allegory also has this very relevant passage:

Once thus enlightened, so to speak, the freed prisoner would no doubt want to return to the cave to free “his fellow bondsmen”. The problem however is that they would not want to be freed: descending back into the cave would require that the freed prisoner’s eyes adjust again, and for a time, he would be inferior at the ludicrous process of identifying shapes on the wall. This would make his fellow prisoners murderous toward anyone who attempted to free them.

Which lines up extremely well with the Gnostic conception of Jesus, that he tried to give us the divine knowledge, gnosis, by which we could free ourselves. But since most people are unwilling or unable to move beyond the everyday shadows cast on the wall into something greater, they crucified Jesus out of fear.