To follow up on my previous posts of quotes on Communicatio in Sacris, I would like to provide some more interesting thoughts on the matter by our illustrious Fathers.
St. John Damascene: “With all our strength, therefore, let us never receive communion from or grant it to heretics; ‘Give not that which is holy unto dogs, saith the Lord, neither cast ye your pearls before swine,’ (Matt. 7:6); lest we become partakers in their dishonor and condemnation.” (Patrologia Graeca, vol. 94, col. 1149, 1152, 1153; Also De Fide Orthodoxa (Exposition of the Orthodox Faith), Book IV, Chapter XIII).
St. Basil the Great, Archbishop of Caesaria in Cappodocia: “As for all those who pretend to confess sound orthodox Faith, but are in communion with people who hold a different opinion, if they are forewarned and still remain stubborn [if we have admonished them once or twice but they still remain obstinate in their heresy], you must not only not be in communion with them, but you must not even call them brothers.” (Patrologia Orientalis, Vol. 17, p. 303)
Pope St. Gregory the Great, Dialogues (c. 593 A.D.): “Rather ought every one to submit to death, than to receive the sacrament of communion from the hand of a heretic.” (Quoted by Gratian, Decretum, 42. xxiv. q. 1)
Pope St. Leo the Great, Sermon 129: “Wherefore, since outside the Catholic Church there is nothing perfect, nothing undefiled, the Apostle declaring that “all that is not of faith is sin” (Romans 14:23), we are in no way likened with those who are divided from the unity of the Body of Christ; we are joined in no communion.”
Origen: “If you eat the words of God in the Church, and also eat them in the ѕуηαgσgυє of the Jєωs, you transgress the commandment which says: “In one House shall it be eaten.” (Exodus 12:46).”
St. Cyril of Alexandria, On Leviticus 17:3: “It is therefore unlawful, and a profanation, and an act the punishment of which is death, to love to associate with unholy heretics, and to unite oneself to their communion.”
St. Athanasius the Great: “We are bound to refrain from communing with those whose opinions we abhor.” (Patrologia Græca, vol. XXVI, col. 1188B (“To Those Who Practice the Solitary Life and Who Are Established in Faith in God”)
St. Theodore the Studite (759-826 A.D.): “If anyone should not number with the other heresies the heresy which... say that communion with heretics is a matter of indifference, he is a heretic.” Another translation says: “If anyone should... say that fellowship with these people is a matter of indifference, he is a heretic.” (Patrologia Graeca, vol. XCIX, col. 352B (“First Refutation of the Iconclasts,” s. 20)
St. Martin of Tours: “I grieve for having been, if only for an hour, in communion with guilty men.” (The Life of Martin, by Sulpitius Severus)
St. Thomas Aquinas, Summa Theologica, Supplement, Q. 23, Art. 2: “An excommunicated person [such as a heretic] is banished from communion. Therefore whoever communicates with him leaves the communion of the Church [commits schism]: and hence he seems to be excommunicated.”
The Didache or Teaching of the Twelve Apostles, Chapter 9, On The Eucharist (c. 60-100 A.D.): “Now concerning the Eucharit. But let no one eat or drink of your Eucharist, unless they have been baptized into the name of the Lord [and shares the same faith]; for concerning this also the Lord has said, “Give not that which is holy to the dogs” (Matthew 7:6).” (The Didache, The Teachings of the Twelve Apostles, Early Christian Treatise)
Council of Laodicea, Canon 9: “Those who are members of the Church are not to be permitted to go into the cemeteries of any of the heretics for the purpose of prayer or veneration. If they do, they are to be excommunicated.” Another version says: “The members of the Church are not allowed to meet in the cemeteries, nor attend the so-called martyries of any of the heretics, for prayer or service; but such as so do, if they be communicants, shall be excommunicated for a time; but if they repent and confess that they have sinned they shall be received.”
Council of Laodicea, Canon 33: “No one shall join in prayers with heretics or schismatics.”
Apostolic Constitutions, Canon 65: “If any one, either of the clergy or laity, enters into a ѕуηαgσgυє of the Jєωs or heretics to pray, let him be deprived and suspended.”
“How does a Catholic sin against faith? A Catholic sins against Faith by Apostasy, heresy, indifferentism and by taking part in non-Catholic worship.” (Catechism of the Council of Trent, Catechism [attributed to] Pope St. Pius X and The Baltimore Catechism)
“It is unlawful for the faithful to assist in any active manner, or to take part in the sacred services of non-Catholics.” (1917 Code of Canon Law, Canon 1258.1)
“You help the ungodly, and you are joined in friendship with those who hate the Lord; and therefore you did indeed deserve the wrath of the Lord.” (II Paralipomenon 19:2)
“I will not communicate with the choicest of them... Depart from me, ye malignant ones!” (Psalm 140:4; 118:115)
“A man that is a heretic, after the first and second admonition, avoid: Knowing that he, that is such an one, is subverted, and sinneth, being condemned by his own judgment.” (Titus 3:10-11)
“Are heretics and schismatics excommunicated? Yes; they have no part in the Communion of the Saints.” (St. Thomas Aquinas, Catechism of the Summa)
“St. Anthony the Abbot would not speak to a heretic, except to exhort him to the true faith; and he drove all heretics from his mountain, calling them venomous serpents.” (St. Athanasius on the life of St. Anthony the Hermit)
“Saint Peter and Paul have loathed heretics, and in their Epistles have warned us to avoid them.” (St. Cyprian)
“I will not pray with you, nor shall you pray with me; neither will I say “Amen” to your prayers, nor shall you to mine!” (Blessed Margaret Clitherow)
From the Life of St. John the Almsgiver - Admonition against taking communion from heretics: “Another thing the blessed man taught and insisted upon with all was never on any occasion whatsoever to associate with heretics and, above all, never to take the Holy Communion with them, ‘even if’, the blessed man said:
“You remain without communicating all your life, if through stress of circuмstances you cannot find a community of the Catholic Church. For if, having legally married a wife in this world of the flesh, we are forbidden by God and by the laws to desert her and be united to another woman, even though we have to spend a long time separated from her in a distant country, and shall incur punishment if we violate our vows, how then shall we, who have been joined to God through the orthodox faith and the Catholic Church–as the apostle says: ‘I espoused you to one husband that I might present you as a pure virgin to Christ’ (2 Cor. 11:2) -- how shall we escape from sharing in that punishment which in the world to come awaits heretics, if we defile the orthodox and holy faith by adulterous communion with heretics?” For ‘communion’, he said, “has been so called because he who has ‘communion’ has things in common and agrees with those with whom he has ‘communion’. Therefore I implore you earnestly, children, never to go near the oratories of the heretics in order to communicate there.” (Three Byzantine Saints, “The Life of Saint John the Almsgiver”, Translators: Elizabeth Dawes & Norman H. Baynes, St. Vladimir’s Seminary Press, Crestwood: 1977; p. 251)