15 But if we consider what the apostles thought about heretics, we shall find that they, in all their epistles, execrated and detested the sacrilegious wickedness of heretics. For when they say that “their word creeps as a canker,” how is such a word as that able to give remission of sins, which creeps like a canker to the ears of the hearers? And when they say that there can be no fellowship between righteousness and un-righteousness, no communion between light and darkness, how can either darkness illuminate, or unrighteousness justify? And when they say that “they are not of God, but are of the spirit of Antichrist,” how can they transact spiritual and divine matters, who are the enemies of God, and whose hearts the spirit of Antichrist has possessed? Wherefore, if, laying aside the errors of human dispute, we return with a sincere and religious faith to the evangelical authority and to the tradition, we shall perceive that they may do nothing towards conferring the ecclesiastical and saving grace, who, scattering and attacking the Church of Christ, are called adversaries by Christ Himself, but by His apostles, Antichrists.
Source: The Epistles of Cyprian (New Advent)