The LORD teaches us that many "ravening wolves shall come in sheep's clothing." The sheep's clothings that 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 are the antichrists, both now and evermore, but the men who rebel against Christ? Heresies, at eh present time, will no less rend the church boy their perversion of doctrine, that 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 steadfast 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 contrarily 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.

Tertullian