In the next place, since he reproaches us with the existence of heresies in Christianity as being a ground of accusation against it, saying that “when Christians had greatly increased in numbers, they were divided and split up into factions, each individual desiring to have his own party;” and further, that “being thus separated through their numbers, they confute one another, still having, so to speak, one name in common, if indeed they still retain it. And this is the only thing which they are yet ashamed to abandon, while other matters are determined in different ways by the various sects.”
In reply to which, we say that heresies of different kinds have never originated from any matter in which the principle involved was not important and beneficial to human life. For since the science of medicine is useful and necessary to the human race, and many are the points of dispute in it respecting the manner of curing bodies, there are found, for this reason, numerous heresies confessedly prevailing in the science of medicine among the Greeks, and also, I suppose, among those barbarous nations who profess to employ medicine.
And, again, since philosophy makes a profession of the truth, and promises a knowledge of existing things with a view to the regulation of life, and endeavours to teach what is advantageous to our race, and since the investigation of these matters is attended with great differences of opinion, innumerable heresies have consequently sprung up in philosophy, some of which are more celebrated than others. Even Judaism itself afforded a pretext for the origination of heresies, in the different acceptation accorded to the writings of Moses and those of the prophets.
So, then, seeing Christianity appeared an object of veneration to men, not to the more servile class alone, as Celsus supposes, but to many among the Greeks who were devoted to literary pursuits, there necessarily originated heresies—not at all, however, as the result of faction and strife, but through the earnest desire of many literary men to become acquainted with the doctrines of Christianity. The consequence of which was, that, taking in different acceptations those discourses which were believed by all to be divine, there arose heresies, which received their names from those individuals who admired, indeed, the origin of Christianity, but who were led, in some way or other, by certain plausible reasons, to discordant views.
And yet no one would act rationally in avoiding medicine because of its heresies; nor would he who aimed at that which is seemly entertain a hatred of philosophy, and adduce its many heresies as a pretext for his antipathy. And so neither are the sacred books of Moses and the prophets to be condemned on account of the heresies in Judaism.
Source: Against Celsus (New Advent)