Cyprian of Carthage, On the Unity of the Church 10

From this, heresies have often arisen and still arise, while a perverse mind lacks peace and faithless discord fails to keep unity. Yet the Lord allows and endures these things, since the freedom of His own will remains, so that while He examines the discernment of truth in our hearts and minds, the unshaken faith of the tested may shine in clear light. Through the Apostle the Holy Spirit forewarns and says: "There must be heresies, so that those who are approved may be revealed among you" (1 Corinthians 11:19). Thus the faithful are proved, thus the faithless are exposed. Thus even now, before the day of judgment, the souls of the just and the unjust are already divided here, and the chaff is separated from the wheat. These are the ones who push themselves forward among the reckless without divine appointment, who set themselves up as prelates without any law of ordination, who take the title of bishop for themselves though no one has given them the episcopate. The Holy Spirit marks them in the Psalms as sitting in the chair of pestilence, as pests and plagues of the faith, deceiving with a serpent's mouth and crafting the corruption of truth, vomiting lethal poisons with pestilential tongues; their speech creeps like cancer, and their dealings pour mortal poison into the breasts and hearts of individuals.