Cyprian of Carthage, On the Unity of the Church 17

The excessive and abrupt perfidy of many must not move or disturb us; rather, with the truth of the matter already foretold, it should strengthen our faith. Because certain men have begun to be such as were predicted beforehand, the rest of the brothers must beware of such people, since these things were also foretold, with the Lord instructing and saying, "But you, beware; behold, I have foretold everything to you" (Mark 13:23). Brothers, I beg you, avoid such men and bar pernicious talk from your side and ears as if it were a contagion of death, just as it is written: "I have hedged my ears with thorns; I will not listen to a wicked tongue" (Sirach 28:28). And again: "Bad company corrupts good character" (1 Corinthians 15:33). The Lord teaches and admonishes us to withdraw from such people. He says, "They are blind guides of the blind" (Matthew 15:14), and, "If the blind lead the blind, both will fall into a pit" (Matthew 15:14). Anyone separated from the Church must be shunned and avoided. Such a person is perverse and sins, and stands self-condemned. Does he think he is with Christ who acts against Christ's priests and separates himself from the fellowship of Christ's clergy and people? He bears arms against the Church and rebels against God's ordinance. He is an enemy of the altar, a rebel against Christ's sacrifice, faithless in faith, sacrilegious in religion, a disobedient servant, an impious son, a hostile brother. Having despised and abandoned the bishops and God's priests, he dares to set up another altar, to offer another prayer with illicit voices, and to profane the truth of the Lord's sacrifice through false sacrifices. He does not realize that whoever resists God's ordination is punished by divine retribution because of the audacity of his temerity.