Tertullian, Prescription against Heretics 4

We should rather keep in mind both the Lord's pronouncements and the apostolic letters, which foretold future heresies to us and defined them as things to be avoided (Matt 7:15); and just as we do not fear that those heresies exist, so we should not be surprised that they can do the very thing for which they are to be avoided. The Lord teaches that many will come in sheep's clothing as rapacious wolves. What are these sheepskins, if not the external surface of the Christian name? Who are the rapacious wolves, if not cunning minds and spirits lurking outwardly to harass Christ's flock? Who are the false prophets, if not false preachers? Who are the false apostles, if not adulterous evangelists? Who are the antichrists, both now and always, if not rebels against Christ? Heresies exist now, troubling the Church with the perversity of their doctrines no less than the Antichrist will persecute it then with the atrocity of his persecutions; the only difference is that persecution makes martyrs, while heresy makes only apostates. And therefore heresies also had to exist, so that the approved might be revealed — both those who stand firm in persecutions and those who do not deviate into heresies. For it does no good to regard as approved those who change their faith into heresy: just as, on the other hand, they interpret the matter for themselves, because He said elsewhere: "Examine all things; hold fast to what is good" (1 Thess 5:21). As if it were not possible, after examining all things poorly, to fall by error into the choice of some evil.