Tertullian

The character of the times in which we live is such as to call forth from us even this admonition, that we ought not to be astonished at the heresies (which abound) neither ought their existence to surprise us, for it was foretold that they should come to pass; nor the fact that they subvert the faith of some, for their final cause is, by affording a trial to faith, to give it also the opportunity of being "approved." Groundless, therefore, and inconsiderate is the offence of the many who are scandalized by the very fact that heresies prevail to such a degree.

And therefore "heresies must needs be in order that they which are approved might be made manifest, both those who remained stedfast under persecution, and those who did not wander out of their way into heresy.

Now, that which he subjoins to evil things, he of course confesses to be itself an evil; and all the greater, indeed, because he tells us that his belief of their schisms and dissensions was grounded on his knowledge that "there must be heresies also." For he shows us that it was owing to the prospect of the greater evil that he readily believed the existence of the lighter ones; and so far indeed was he from believing, in respect of evils (of such a kind), that heresies were good, that his object was to forewarn us that we ought not to be surprised at temptations of even a worse stamp, since (he said) they tended "to make manifest all such as were approved; " in other words, those whom they were unable to pervert.

It was indeed necessary that there should be heresies; and yet it does not follow from that necessity, that heresies are a good thing.

Nor do I risk contradiction in saying that the very Scriptures were even arranged by the will of God in such a manner as to furnish materials for heretics, inasmuch as I read that "there must be heresies, which there cannot be without the Scriptures.