Now, if these arguments hold good, why should we not defend, in the same way, the existence of heresies in Christianity? And respecting these, Paul appears to me to speak in a very striking manner when he says, “For there must be heresies among you, that they who are approved may be made manifest among you.” For as that man is “approved” in medicine who, on account of his experience in various (medical) heresies, and his honest examination of the majority of them, has selected the preferable system—and as the great proficient in philosophy is he who, after acquainting himself experimentally with the various views, has given in his adhesion to the best,— so I would say that the wisest Christian was he who had carefully studied the heresies both of Judaism and Christianity.
Whereas he who finds fault with Christianity because of its heresies would find fault also with the teaching of Socrates, from whose school have issued many others of discordant views. Nay, the opinions of Plato might be chargeable with error, on account of Aristotle's having separated from his school, and founded a new one—on which subject we have remarked in the preceding book. But it appears to me that Celsus has become acquainted with certain heresies which do not possess even the name of Jesus in common with us. Perhaps he had heard of the sects called Ophites and Cainites, or some others of a similar nature, which had departed in all points from the teaching of Jesus. And yet surely this furnishes no ground for a charge against the Christian doctrine.
Source: Against Celsus (New Advent)