II. The seductions of heretics capture the unwary.
There are some “in sheep's clothing, but inwardly they are ravening wolves:” whom we know by their fruit. These men seem indeed at first to be of us, but they are not of us: “for if they had been of us, they would no doubt have continued with us.” But when they have spewed out their impiety, throwing out the guile that is in them, and seizing the weaker ones, and those who have their senses unpractised in the divine utterances, they carry them along with themselves to destruction, wresting and doing despite to the Father.' doctrines, just as they do the Holy Scriptures also to their own destruction: whom we must be forewarned of and take heed lest some should be misled by their wickedness and shaken in their firmness. “For they have sharpened their tongues like serpents: adder's poison is under their lips,” as the prophet has cried out about them.
Source: Letters (New Advent)