Patriarch of Antioch Theodore Balsamon ca. 12th cent.
Question: Shall one perform priestly rites or pray together without danger with heretics, namely Jacobites and Nestorians, in their churches or even our own, or might one share a common table with them, or perform sponsorship at holy baptism, or perform memorial services of the departed, or commune of the Divine Sanctified Elements with them? For the areas difficulties create many such things, and I seek what one must do.
“Do not give the holy things to the dogs,” our Lord and God has said, nor “cast pearls before swine.” Indeed, on this account account Canon 64 of the Holy Apostles, the heralds of God, also states, “if any clergyman or layman might enter an assembly of the Jews or heretics to pray, let him be defrocked and excommunicated.” Canon 33 of the Council in Laodicea, but indeed also 6 and 34, states the following: “Concerning not permitting heretics to enter into a house of God while they remain in heresy,” because one must not pray with a heretic or schismatic, “a Christian must not abandon Christ’s martyrs and depart for false martyrs, namely, heretical ones or those that the aforementioned heretics produced. For these are estranged from God. Therefore, let those departing to them be anathematized.” Indeed, on this account we also decided that both clergy and laity are subject not only to excommunication and defrocking when they pray together in a church of Orthodox or heretics or whenever they pray together as clergy, or even share a meal together, but also shall they be punished in a more severe way, according to the provisions of the cited divine canons. For the difficulties of areas, and the increase of heretics, did not change the soundness of the Orthodox Faith. (Canonical Questions of the Most Holy Patriarch of Alexandria, Lord Markos, and the Answers for them by the Most Holy Patriarch of Antioch, Lord Theodoros Balsamon: Question 15. Viscuso, “A Guide to the Church Under Islam” pp. 82-84)