The Prescription Against Heretics by Tertullian
Chapter V.--Heresy, as Well as Schism and Dissension, Disapproved by St. Paul, Who Speaks of the Necessity of Heresies, Not as a Good, But, by the Will of God, Salutary Trials for Training and Approving the Faith of Christians.
Moreover, when he blames dissensions and schisms, which undoubtedly are evils, he immediately adds heresies likewise. 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. In short, since the whole passage points to the maintenance of unity and the checking of divisions, inasmuch as heresies sever men from unity no less than schisms and dissensions, no doubt he classes heresies under the same head of censure as he does schisms also and dissensions. And by so doing, he makes those to be |not approved,| who have fallen into heresies; more especially when with reproofs he exhorts men to turn away from such, teaching them that they should |all speak and think the selfsame thing,| the very object which heresies do not permit.