The Prescription Against Heretics by Tertullian
Chapter II.--Analogy Between Fevers and Heresies Heresies Not to Be Wondered At: Their Strength Derived from Weakness of Men's Faith. They Have Not the Truth. Simile of Pugilists and Gladiators in Illustration.
Taking the similar case of fever, which is appointed a place amongst all other deadly and excruciating issues (of life) for destroying man: we are not surprised either that it exists, for there it is, or that it consumes man, for that is the purpose of its existence. In like manner, with respect to heresies, which are produced for the weakening and the extinction of faith, since we feel a dread because they have this power, we should first dread the fact of their existence; for as long as they exist, they have their power; and as long as they have their power, they have their existence. But still fever, as being an evil both in its cause and in its power, as all know, we rather loathe than wonder at, and to the best of our power guard against, not having its extirpation in our power. Some men prefer wondering at heresies, however, which bring with them eternal death and the heat of a stronger fire, for possessing this power, instead of avoiding their power when they have the means of escape: but heresies would have no power, if (men) would cease to wonder that they have such power. For it either happens that, while men wonder, they fall into a snare, or, because they are ensnared, they cherish their surprise, as if heresies were so powerful because of some truth which belonged to them. It would no doubt be a wonderful thing that evil should have any force of its own, were it not that heresies are strong in those persons who are not strong in faith. In a combat of boxers and gladiators, generally speaking, it is not because a man is strong that he gains the victory, or loses it because he is not strong, but because he who is vanquished was a man of no strength; and indeed this very conqueror, when afterwards matched against a really powerful man, actually retires crest-fallen from the contest. In precisely the same way, heresies derive such strength as they have from the infirmities of individuals -- having no strength whenever they encounter a really powerful faith.