Cyprian of Carthage, Three Books of Testimonies Against the Jews III.79: That we must not speak with heretics

To Titus: “A man that is an heretic, after one rebuke avoid; knowing that one of such sort is perverted, and sinneth, and is by his own self condemned.” Of this same thing in the Epistle of John: “They went out from among us, but they were not of us; for if they had been of us, they would doubtless have remained with us.” Also in the second to Timothy: “Their word doth creep as a canker.”