The Prescription Against Heretics by Tertullian
Chapter IV.--Warnings Against Heresy Given Us in the New Testament Sundry Passages Adduced. These Imply the Possibility of Falling into Heresy.
But let us rather be mindful of the sayings of the Lord, and of the letters of the apostles; for they have both told us beforehand that there shall be heresies, and have given us, in anticipation, warnings to avoid them; and inasmuch as we are not alarmed because they exist, so we ought not to wonder that they are capable of doing that, on account of which they must be shunned. The Lord teaches us that many |ravening wolves shall come in sheep's clothing.| Now, what are these sheep's clothing's, but the external surface of the Christian profession? Who are the ravening wolves but those deceitful senses and spirits which are lurking within to waste the flock of Christ? Who are the false prophets but deceptive predictors of the future? Who are the false apostles but the preachers of a spurious gospel? Who also are the Antichrists, both now and evermore, but the men who rebel against Christ? Heresies, at the present time, will no less rend the church by their perversion of doctrine, than will Antichrist persecute her at that day by the cruelty of his attacks, except that persecution make seven martyrs, (but) heresy only apostates. 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. For the apostle does not mean that those persons should be deemed approved who exchange their creed for heresy; although they contrariously interpret his words to their own side, when he says in another passage, |Prove all things; hold fast that which is good;| as if, after proving all things amiss, one might not through error make a determined choice of some evil thing.