Heresies have often arisen and still arise because of this, that disgruntled minds will quarrel, or disloyal trouble-makers will not keep the unity. But these things the Lord allows and endures, leaving man’s freedom unimpaired, so that when our minds and hearts are tested by the touchstone of truth, the unswerving faith of those who are approved may appear in the clearest light. This is foretold by the Holy Spirit through the Apostle when he says: There must also be heresies, that those approved may be manifest among you. Thus are the faithful proved, thus the faithless discovered; thus too even before the day of judgment, already here below, the souls of the just and unjust are distinguished, and the wheat is separated from the chaff. This explains why certain people, backed by their hot-headed associates, seize authority for themselves without any divine sanction, making themselves into prelates regardless of the rules of appointment, and, having no one to confer the episcopate upon them, assume the title of Bishop on their own authority. In the Psalms the Holy Spirit describes these men as sitting in the chair of pestilence; they are pests and plagues to the faith, snake-tongued deceivers, skilled corruptors of the truth, spewing deadly venom from their poisonous fangs; whose speech spreads like a canker; whose preaching injects a fatal virus in the hearts and breasts of all.
[From The Unity of the Catholic Church by St. Cyprian]