Jamieson, Fausset and Brown on Acts 20:29-30

29-30. after my departing shall grievous wolves enter in among you--Two classes of coming enemies are here announced, the one more external to themselves, the other bred in the bosom of their own community; both were to be teachers, but the one, "grievous wolves," not sparing, that is, making a prey of the flock; the other (Ac 20:30), simply sectarian "perverters" of the truth, with the view of drawing a party after them. Perhaps the one pointed to that subtle poison of Oriental Gnosticism which we know to have very early infected the Asiatic churches; the other to such Judaizing tendencies as we know to have troubled nearly all the early churches. See the Epistles to the Ephesians, Colossians, and Timothy, also those to the seven churches of Asia (Re 2:1-3:22). But watchfulness against all that tends to injure and corrupt the Church is the duty of its pastors in every age.