Cyprian of Carthage, Epistle 63.5 (Cup of the Lord)

The lapsed must recognize the magnitude of their offense, yet they must not stop beseeching the Lord, nor must they abandon the Catholic Church, which the Lord alone established as one. Instead, they should persist in acts of satisfaction and plead for the Lord's mercy, knocking at the Church so that they may be received back where they once stood and return to Christ, from whom they turned away. They must not listen to those who deceive themselves with fallacious and mortal seduction, since it is written: "Let no one deceive you with empty words" (Ephesians 5:6). For this reason the wrath of God comes upon the sons of disobedience. Therefore, do not become sharers with them. Consequently, no one should associate with the disobedient, those who do not fear God, and those who withdraw completely from the Church. But if anyone grows impatient in beseeching the offended Lord, refuses to obey us, and instead follows the desperate and the lost, he will have only himself to blame when the day of judgment arrives. For how will he be able to beseech the Lord on that day—he who previously denied Christ and now also denies the Church of Christ, and, by not obeying bishops who are sound, whole, and living, has made himself a companion and sharer of the dying? I pray that you, my most dear and longed-for brothers, may always fare well.