21 But for the rest, let our most beloved brethren firmly decline, and avoid the words and conversations of those whose word creeps onwards like a cancer; as the apostle says, “Evil communications corrupt good manners.” And again: “A man that is an heretic, after one admonition, reject: knowing that he that is such is subverted, and sins, being condemned of himself.” And the Holy Spirit speaks by Solomon, saying, “A perverse man carries perdition in his mouth; and in his lips he hides a fire.” Also again, he warns us, and says, “Hedge in your ears with thorns, and hearken not to a wicked tongue.” And again: “A wicked doer gives heed to the tongue of the unjust; but a righteous man does not listen to lying lips.” And although I know that our brotherhood there, assuredly fortified by your foresight, and besides sufficiently cautious by their own vigilance, cannot be taken nor deceived by the poisons of heretics, and that the teachings and precepts of God prevail with them only in proportion as the fear of God is in them; yet, even although needlessly, either my solicitude or my love persuaded me to write these things to you, that no commerce should be entered into with such; that no banquets nor conferences be entertained with the wicked; but that we should be as much separated from them, as they are deserters from the Church; because it is written, “If he shall neglect to hear the Church, let him be unto you as a heathen man and a publican.” And the blessed apostle not only warns, but also commands us to withdraw from such. “We command you,” he says, “in the name of Jesus Christ our Lord, that you withdraw yourselves from every brother that walks disorderly, and not after the tradition which he received of us.” There can be no fellowship between faith and I faithlessness. He who is not with Christ, who is an adversary of Christ, who is hostile to His unity and peace, cannot be associated with us. If they come with prayers and atonements, let them be heard; if they heap together curses and threats, let them be rejected. I bid you, dearest brother, ever heartily farewell.
Source: The Epistles of Cyprian (New Advent)