“And the Lord said to Moses and Aaron: This is the law of the Passover: no stranger shall eat of it. And every slave or servant bought with money - him thou shalt circumcise, and then shall he eat of it. A sojourner or hireling shall not eat of it. In one house shall it be eaten.” [Exodus 12:43-46.]

St. Apraphat of Syria writes that the “one house” in which the Passover is to be eaten is “the Church of Christ”, and that just as the slave could not eat the Passover unless he was circumcised, so the sinner “comes to Baptism, the true Circumcision, and is joined to the People of God, and communicates in the Body and Blood of Christ”. [Demonstrations 12, 525.8, 525.12.]

St. John Chrysostom writes: “Let no-one communicate who is not of the disciples. Let no Judas receive, lest he suffer the fate of Judas... I would give up my life rather than impart of the Lord’s Blood to the unworthy; and I will shed my own blood rather than give such awful Blood contrary to what is right.” [Homilies on Matthew, 83.6.]

St. John the Almsgiver said: “We shall not escape sharing in that punishment which, in the world to come, awaits heretics, if we defile Orthodoxy and the holy Faith by adulterous communion with heretics.” [The Life of St. John the Almsgiver.]

St. John of Damascus writes: “With all our strength let us beware lest we receive Communion from or give it to heretics. ‘Give not what is holy to the dogs,’ says the Lord. ‘Neither cast ye your pearls before swine’, lest we become partakers in their dishonour and condemnation.” [Exposition of the Orthodox Faith, IV, 13.]

“Holy things are for the holy!” [The Divine Liturgy.]

“And the Lord said to Joshua, Rise up: why hast thou fallen upon thy face? The people has sinned, and transgressed the covenant which I made with them; they have stolen from the accursed things (Greek: anathema), and put it into their store. And the children of Israel will not be able to stand before their enemies, for they have become an accursed thing (anathema); I will no longer be with you, unless ye remove the accursed thing (anathema) from yourselves.” [Joshua 7:10-11.]

“Let any Bishop, or Presbyter, or Deacon that merely joins in prayer with heretics be suspended, but if he has permitted them to perform any service as clergymen, let him be deposed.” [Apostolic Canon 45.]

“Let any clergyman or layman who enters a synagogue of Jews, or of heretics, to pray be both deposed and excommunicated.” [Apostolic Canon 65.]

“Concerning the necessity of not permitting heretics to come into the house of God, so long as they persist in their heresy.” [Canon 6 of the Council of Laodicea.]

“That one must not accept the blessings of heretics, which are rather misfortunes than blessings.” [Canon 32 of the Council of Laodicea.]

“That one must not join in prayer with heretics or schismatics.” [Canon 33 of the Council of Laodicea.]

Saint Cyril of Jerusalem says, “There is one Lord, one Faith, one baptism [Eph 4:5], and this is what we believe with all our soul, with all our heart, and with all our mind, that there is salvation in none other, except in Jesus Christ the Nazarene [Acts 4: 10-12]. And this is what we piously believe and what we follow. And we also recognize that works without true doctrine are not accepted by God, neither is true doctrine without works accepted by God” [St. Cyril of Jerusalem, Illumination of the Catechumens, Section 4, #2].

St. Basil the Great teaches us: “As for all those who pretend to confess the sound Orthodox Faith, but are in communion with people who hold a different opinion, if they are forewarned and still remain stubborn, you must not only not be in communion with them, but you must not even call them brothers.” (*Patr, Orientalis*, Vol 17, p. 303)

...But as for those who...sever themselves from communion with their president, that is, because he publicly preaches heresy and with bared head teaches it in the Church, such persons are not only not subject to canonical penalty..., but are worthy of due honour among the Orthodox.  For not bishops, but false bishops and false teachers have they condemned, and they have not fragmented the Church’s unity with schism, but from schisms and divisions have they earnestly sought to deliver the Church. (Canon XV of the First-Second Council of Constantinople)

All the teachers of the Church, all the Councils, and all the Divine Scriptures, exhort us to flee those who uphold other doctrines and to separate from communion with them. (Confession of Faith, XII, 304, Saint Mark of Ephesus)

St. Maximus the Confessor said: “Even if the whole universe holds communion with the [heretical] patriarch, I will not communicate with him. For I know from the writings of the holy Apostle Paul: the Holy Spirit declares that even the angels would be anathema if they should begin to preach another Gospel, introducing some new teaching.” [The Life of St. Maximus the Confessor.]

With a great voice, Saint John Chrysostom declared that not only heretics, but also they who hold communion with them are enemies of God. (Saint Theodore the Studite , Letter to the Abbot Theophilus)

Concerning the Faith, the heretics were totally shipwrecked; but as for the others, even if in their thinking they did not founder, nonetheless, because of their communion with heresy, they too were destroyed with the others. (Saint Theodore the Studite, Letter to the Patriarch of Jerusalem [PG 99, 1164])

You told me that you feared to tell your presbyter not to commemorate the heresiarch; even though I will not presume to say anything about this to you for the present, except that the communion is defiled simply by commemorating him, even if he who is commemorating is Orthodox. (Ibid.)

“Guard yourselves from soul-destroying heresy, communion with which is alienation from Christ.” [St. Theodore the Studite, P.G. 99.1216.]

“The divine and sacred canons say: ‘He who has communion with an excommunicate, let him be excommunicated, as overthrowing the rule of the Church.’ And again: ‘He who receives a heretic is subject to the same indictment...’ The great apostle and evangelist John says: ‘If anyone comes to you and does not bring this teaching with him, do not greet him and do not receive him into your house; for he who greets him communicates with his evil deeds’ (II John 10-11). If we are forbidden merely to greet him on the way, and if inviting him into our house is prohibited, how can it be otherwise not in a house, but in the temple of God, in the sanctuary at the mystical and terrible Supper of the Son of God... Whoever belches out the commemoration of him who has been worthily cut off by the Holy Spirit for his arrogance towards God and the Divine things, becomes for that reason an enemy of God and the Divine things.” [From an Epistle of the Martyred Fathers of the Holy Mountain to Emperor Michael Palaeologus against the heretical Patriarch John Beccus of Constantinople.]

“Be not unequally yoked together with unbelievers. For what partnership have righteousness and iniquity? Or what fellowship has light with darkness? What accord has Christ with Belial? What agreement has the temple of God with idols? For we are the temple of the living God.” [II Corinthians 6:14-16.]

“Come out of her, My people, lest you take part in her sins, lest you share in her plagues.” [Revelation 18:4.]

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 circumstances you cannot find a community of the Catholic [Orthodox] 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.’