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St. Cyril of Alexander, 5^th century: “It is therefore unlawful and a profanation, and an act the punishment of which is death, to associate with unhallowed heretics and to unite oneself to their communion.”

Pope St. Leo the Great, 5^th century: “It is at the peril of his soul for any one of them [heretics] who has fallen away from [the papacy] into a sect of heretics and schismatics…to be received into Catholic communion after coming to his senses without making legitimate and express satisfaction.”

Pope St. Pelagius, 6^th century: “For there is no crime more hated or despised than to communicate with schismatics…For anyone who does this is a reprobate and ceases to be part of the Church.”

Pope St. Gregory the Great, 6^th century: “Rather ought everyone submit to death than to receive the sacrament of communion from the hand of a heretic.”

St. John Damascene, 8^th century: “Let us never receive communion from or grant it to heretics…lest we become partakers in their dishonor and condemnation.”

St. Thomas Aquinas, Pt. II-II, Q. 39, Art. 3: “Jurisdiction…does not remain in heretics and schismatics; and consequently they neither absolve, nor excommunicate nor grant indulgences, nor do anything of the kind, and if they do it is invalid.”

Pope Paul IV, 16^th century: “On no account [may] you go to the churches of heretics, or hear their sermons, or join in their rites lest ye incur the wrath of God.”

Pope Pius VI, 18^th century; “Charitas”: “Keep away from all intruders, whether called Archbishops, bishops or parish priests; do not hold communion with them especially in divine worship.”

http://betrayedcatholics.com/wpcms/articles/a-catholics-course-of-study/traditionalist-heresies-and-errors/true-status-of-schismatic-priests-and-bishops-ignored/status-of-those-ordainedconsecrated-by-schismatics/