Ante-Nicene Fathers/Volume V/Cyprian/The Treatises of Cyprian/Three Books of Testimonies Against the Jews/Book III/Part 78
78. 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.”[1] 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.”[2] Also in the second to Timothy: “Their word doth creep as a canker.”[3]