“What has Athens to do with Jerusalem?” thundered the church father Tertullian at the end of the second century AD, and the answer implied was: nothing. The triumph of Christianity by the time of Constantine created a radical break with many of the beliefs and practices of the classical past. In the Christian world, classical texts were still read, Greek and Latin were still spoken; but the claims to knowledge found in classical philosophy were not only disowned but even (by some zealots) considered a form of temptation held out by the devil. Where Christianity and its insistence on faith represented “the folly of the wise,” the writings of the philosophers were, in turn, “the wisdom of fools.” Christian thinkers who borrowed from them or tried to reconcile Christianity and Plato had to tread carefully indeed. This lecture explores features of the clash of classical philosophy and the Christian doctrine from late antiquity to the Renaissance and the long-lasting effects of the idea Credo quia absurdum est: “I believe, because it is absurd.” Along the way, we will touch on Socrates, free will, human sexuality, the use of allegory to redeem pagan writing, and, of course, what it means that our intellectual tradition has a radical break caused by the advent of a salvific religion.