Plato in his Republic is arguing that righteousness is often praised for the rewards it brings—honour, popularity, and the like—but that to see it in its true nature we must separate it from all these, strip it naked. He asks us therefore to imagine a perfectly righteous man treated by all around him as a monster of wickedness. We must picture him, still perfect, while he is bound, scourged, and finally impaled (the Persian equivalent of crucifixion). At this passage a Christian reader starts and rubs his eyes. What is happening? Yet another of these lucky coincidences? But presently he sees that there is something here which cannot be called luck at all….
Plato is talking, and knows he is talking, about the fate of goodness in a wicked and misunderstanding world. But that is not something simply other than the Passion of Christ. It is the very same thing of which that Passion is the supreme illustration. If Plato was in some measure moved to write of it by the recent death—we may almost say the martyrdom—of his master Socrates then that again is not something simply other than the Passion of Christ. The imperfect, yet very venerable, goodness of Socrates led to the easy death of the hemlock, and the perfect goodness of Christ led to the death of the cross, not by chance but for the same reason; because goodness is what it is, and because the fallen world is what it is. If Plato, starting from one example and from his insight into the nature of goodness and the nature of the world, was led on to see the possibility of a perfect example, and thus to depict something extremely like the Passion of Christ, this happened not because he was lucky but because he was wise. If a man who knew only England and had observed that, the higher a mountain was, the longer it retained the snow in early spring, were led on to suppose a mountain so high that it retained the snow all the year round, the similarity between his imagined mountain and the real Alps would not be merely a lucky accident. He might not know that there were any such mountains in reality, just as Plato probably did not know that the ideally perfect instance of crucified goodness which he had depicted would ever become actual and historical. But if that man ever saw the Alps he would not say “What a curious coincidence”. He would be more likely to say “There! What did I tell you?”
C.S. Lewis, “Second Meanings,” Reflections on the Psalms (1958, this excerpt taken from The Essential C.S. Lewis Touchstone, 1998) 399-400.