First, what he did predict. Having experienced some of the depths of the wickedness of men, Plato understood that an absolutely just man would have suffered absolutely at the hands of his neighbors. He says through Glaucon:
Though he do no wrong he must have the repute of the greatest injustice, so that he may be put to the test. ... But let him hold on his course unchangeable even unto death, seeming all his life to be unjust though being just. ... Such being his disposition the just man will have to endure the lash, the rack, chains, the branding iron in his eyes, and finally, after every extremity of suffering, he will be crucified. (Republic II, 361c-e passim)Here Plato's account is practically a prophecy. Although its precision is astonishing, it remains in the natural order, since inspiration is not required to see the logical conclusions of sin.
What Plato did not predict, what he could not have predicted, was that the absolutely just man would live again after his crucifixion and death. St. Justin Martyr points out that there are pagan analogues to Christ's resurrection -- Odysseus coming back from the underworld, the rising of the phoenix -- but the Greeks did not imagine that a Jewish man could physically rise from the dead by his own power (see First Apology chs. 18-20). Once again we find that the two central mysteries of Christian faith are the Incarnation and the Resurrection of Jesus Christ.