Eucharistic theology in the Fathers

The Orthodox doctrine of the Eucharist from the apostolic age forward — that the bread and the cup, blessed by the Word and the Spirit through the prayers of the bishop, are the very Body and Blood of the incarnate Lord; that this offering is the one sacrifice of Christ made present, not a new sacrifice; that participation in it unites the faithful to God and to one another; and that no schism, no false belief, no breaking from the bishop can have part in it.

Mt 26:26-29
Matthew · Holy Apostle and Evangelist Matthew · World English Bible (NT)
²⁶ As they were eating, Jesus took bread, gave thanks for it, and broke it. He gave to the disciples and said, “Take, eat; this is my body.” ²⁷ He took the cup, gave thanks, and gave to them, saying, “All of you drink it, ²⁸ for this is my blood of the new covenant, which is poured out for many for the remission of sins. ²⁹ But I tell you that I will not drink of this fruit of the vine from now on, until that day when I drink it anew with you in my Father’s Kingdom.”
The institution of the Eucharist at the Mystical Supper. "Take, eat; this is my body... drink ye all of it; for this is my blood of the new covenant." Read by the Fathers as words of effective consecration, not mere symbol — the same Christ who first spoke them speaks them at every Liturgy through the priest.
Mt 26:26-28 with parallels Mk 14:22-24, Lk 22:19-20, 1 Cor 11:23-26
Jn 6:53-59
John · Holy Apostle and Evangelist John the Theologian · World English Bible (NT)
⁵³ Jesus therefore said to them, “Most certainly I tell you, unless you eat the flesh of the Son of Man and drink his blood, you don’t have life in yourselves. ⁵⁴ He who eats my flesh and drinks my blood has eternal life, and I will raise him up at the last day. ⁵⁵ For my flesh is food indeed, and my blood is drink indeed. ⁵⁶ He who eats my flesh and drinks my blood lives in me, and I in him. ⁵⁷ As the living Father sent me, and I live because of the Father, so he who feeds on me will also live because of me. ⁵⁸ This is the bread which came down out of heaven—not as our fathers ate the manna and died. He who eats this bread will live forever.” ⁵⁹ He said these things in the synagogue, as he taught in Capernaum.
"Most certainly I tell you, unless you eat the flesh of the Son of Man and drink his blood, you don't have life in yourselves." The discourse at Capernaum on the Bread of Life — read by the Orthodox tradition as direct Eucharistic teaching, not as figurative language. The Fathers note that Christ does not soften the saying when many disciples leave.
Jn 6:48-58; cited by St Cyril of Alexandria and the conciliar tradition
1Apol 66.1
The First Apology · Holy Martyr Justin the Philosopher · Ante-Nicene Fathers (Roberts–Donaldson)
And this food is called among us Εὐχαριστία [the Eucharist], of which no one is allowed to partake but the man who believes that the things which we teach are true, and who has been washed with the washing that is for the remission of sins, and unto regeneration, and who is so living as Christ has enjoined. For not as common bread and common drink do we receive these; but in like manner as Jesus Christ our Saviour, having been made flesh by the Word of God, had both flesh and blood for our salvation, so likewise have we been taught that the food which is blessed by the prayer of His word, and from which our blood and flesh by transmutation are nourished, is the flesh and blood of that Jesus who was made flesh. For the apostles, in the memoirs composed by them, which are called Gospels, have thus delivered unto us what was enjoined upon them; that Jesus took bread, and when He had given thanks, said, “This do ye in remembrance of Me, this is My body;” and that, after the same manner, having taken the cup and given thanks, He said, “This is My blood;” and gave it to them alone. Which the wicked devils have imitated in the mysteries of Mithras, commanding the same thing to be done. For, that bread and a cup of water are placed with certain incantations in the mystic rites of one who is being initiated, you either know or can learn.
St Justin Martyr (c. 155 AD): "And this food is called among us Eucharist... we have been taught that the food which is blessed by the prayer of His word, and from which our blood and flesh by transmutation are nourished, is the flesh and blood of that Jesus who was made flesh." The earliest extant non-canonical witness to Eucharistic realism.
St Justin Martyr, First Apology 66
Ign Smyrn 7.1
Ignatius to the Smyrnaeans · Hieromartyr Ignatius the God-bearer of Antioch · Apostolic Fathers, tr. J. B. Lightfoot
They therefore that gainsay the good gift of God perish by their questionings. But it were expedient for them to have love, that they may also rise again.
St Ignatius of Antioch (c. 107 AD), writing against docetic heretics: "They abstain from Eucharist and from prayer, because they confess not the Eucharist to be the flesh of our Saviour Jesus Christ, which flesh suffered for our sins, and which the Father of His goodness raised up." Eucharistic realism is here the test of orthodox Christology itself.
St Ignatius, Epistle to the Smyrnaeans 7
Ign Eph 20.2
Ignatius to the Ephesians · Hieromartyr Ignatius the God-bearer of Antioch · Apostolic Fathers, tr. J. B. Lightfoot
especially if the Lord should reveal aught to me. Assemble yourselves together in common, every one of you severally, man by man, in grace, in one faith and one Jesus Christ, who after the flesh was of David's race, who is Son of Man and Son of God, to the end that ye may obey the bishop and the presbytery without distraction of mind; breaking one bread, which is the medicine of immortality and the antidote that we should not die but live for ever in Jesus Christ.
St Ignatius calls the Eucharist "the medicine of immortality, the antidote that we should not die but live forever in Jesus Christ." The Eucharist not only signifies but effects the immortality of the partaker.
St Ignatius, Epistle to the Ephesians 20
Ign Phld 4.1
Ignatius to the Philadelphians · Hieromartyr Ignatius the God-bearer of Antioch · Apostolic Fathers, tr. J. B. Lightfoot
Be ye careful therefore to obscene one eucharist (for there is one flesh of our Lord Jesus Christ and one cup unto union in His blood; there is one altar, as there is one bishop, together with the presbytery and the deacons my fellow-servants), that whatsoever ye do, ye may do it after God.
"Be ye careful therefore to observe one Eucharist; for there is one flesh of our Lord Jesus Christ, and one cup unto union in His blood; one altar, as there is one bishop together with the presbytery and the deacons." The one Eucharist, the one altar, and the one bishop are inseparable in early Christian ecclesiology.
St Ignatius, Epistle to the Philadelphians 4
Did 9.1
The Didache (Teaching of the Twelve Apostles) · Apostolic Fathers, tr. J. B. Lightfoot
But as touching the eucharistic thanksgiving give ye thanks thus.
The Didache (late 1st / early 2nd c.): "Concerning the Eucharist, give thanks thus..." — and prescribes the prayers over the cup and the broken bread. The earliest liturgical formula for the Eucharist outside the New Testament.
Didache 9-10, 14
Did 14.1
The Didache (Teaching of the Twelve Apostles) · Apostolic Fathers, tr. J. B. Lightfoot
And on the Lord's own day gather yourselves together and break bread and give thanks, first confessing your transgressions, that your sacrifice may be pure.
"On the Lord's own day, having gathered together, break bread and give thanks, having confessed your transgressions in addition... for this is that which was spoken by the Lord, 'In every place and time offer me a pure sacrifice.'" The Eucharist as the prophesied pure offering of Malachi 1:11.
Didache 14
AH 4.18.5
Against Heresies (Adversus Haereses) · Saint Irenaeus of Lyons · Ante-Nicene Fathers (Roberts–Donaldson)
5. Again, giving directions to His disciples to offer to God the first-fruits of His own, created things—not as if He stood in need of them, but that they might be themselves neither unfruitful nor ungrateful—He took that created thing, bread, and gave thanks, and said, “This is My body.” And the cup likewise, which is part of that creation to which we belong, He confessed to be His blood, and taught the new oblation of the new covenant; which the Church receiving from the apostles, offers to God throughout all the world, to Him who gives us as the means of subsistence the first-fruits of His own gifts in the New Testament, concerning which Malachi, among the twelve prophets, thus spoke beforehand: “I have no pleasure in you, saith the Lord Omnipotent, and I will not accept sacrifice at your hands. For from the rising of the sun, unto the going down [of the same], My name is glorified among the Gentiles, and in every place incense is offered to My name, and a pure sacrifice; for great is My name among the Gentiles, saith the Lord Omnipotent;”—indicating in the plainest manner, by these words, that the former people [the Jews] shall indeed cease to make offerings to God, but that in every place sacrifice shall be offered to Him, and that a pure one; and His name is glorified among the Gentiles.
St Irenaeus (c. 180): "The bread, which comes from the earth, having received the invocation of God, is no longer common bread, but Eucharist, consisting of two realities — earthly and heavenly." The Eucharist holds together the created and the divine, just as the incarnate Christ does.
St Irenaeus, Against Heresies IV.18.5
Cat 22.1.1
Catechetical Lectures · Saint Cyril of Jerusalem · Ante-Nicene Fathers (Roberts–Donaldson)
Lecture XXI.
St Cyril of Jerusalem's Mystagogical Lecture on the Eucharist: "Since He has declared and said of the bread, 'This is my body,' who shall dare to doubt any longer? And since He has affirmed and said, 'This is my blood,' who shall ever hesitate, saying that it is not His blood?" The 4th-century catechesis as it was actually preached to the newly baptized in Jerusalem.
St Cyril of Jerusalem, Catechetical Lecture XXII (Mystagogical IV)
GrCat 39.2
The Great Catechism (Oratio Catechetica Magna) · Saint Gregory of Nyssa · Ante-Nicene Fathers (Roberts–Donaldson)
But since the human being is a twofold creature, compounded of soul and body, it is necessary that the saved should lay hold of the Author of the new life through both their component parts. Accordingly, the soul being fused into Him through faith derives from that the means and occasion of salvation; for the act of union with the life implies a fellowship with the life. But the body comes into fellowship and blending with the Author of our salvation in another way. For as they who owing to some act of treachery have taken poison, allay its deadly influence by means of some other drug (for it is necessary that the antidote should enter the human vitals in the same way as the deadly poison, in order to secure, through them, that the effect of the remedy may be distributed through the entire system), in like manner we, who have tasted the solvent of our nature, necessarily need something that may combine what has been so dissolved, so that such an antidote entering within us may, by its own counter-influence, undo the mischief introduced into the body by the poison. What, then, is this remedy to be? Nothing else than that very Body which has been shown to be superior to death, and has been the First-fruits of our life. For, in the manner that, as the Apostle says, a little leaven assimilates to itself the whole lump, so in like manner that body to which immortality has been given it by God, when it is in ours, translates and transmutes the whole into itself. For as by the admixture of a poisonous liquid with a wholesome one the whole draught is deprived of its deadly effect, so too the immortal Body, by being within that which receives it, changes the whole to its own nature. Yet in no other way can anything enter within the body but by being transfused through the vitals by eating and drinking. It is, therefore, incumbent on the body to admit this life-producing power in the one way that its constitution makes possible. And since that Body only which was the receptacle of the Deity received this grace of immortality, and since it has been shown that in no other way was it possible for our body to become immortal, but by participating in incorruption through its fellowship with that immortal Body, it will be necessary to consider how it was possible that that one Body, being for ever portioned to so many myriads of the faithful throughout the whole world, enters through that portion, whole into each individual, and yet remains whole in itself. In order, therefore, that our faith, with eyes fixed on logical probability, may harbour no doubt on the subject before us, it is fitting to make a slight digression in our argument, to consider the physiology of the body. Who is there that does not know that our bodily frame, taken by itself, possesses no life in its own proper subsistence, but that it is by the influx of a force or power from without that it holds itself together and continues in existence, and by a ceaseless motion that it draws to itself what it wants, and repels what is superfluous? When a leathern bottle is full of some liquid, and then the contents leak out at the bottom, it would not retain the contour of its full bulk unless there entered in at the top something else to fill up the vacuum; and thus a person, seeing the circumference of this bottle swollen to its full size, would know that this circumference did not really belong to the object which he sees, but that what was being poured in, by being in it, gave shape and roundness to the bulk. In the same way the mere framework of our body possesses nothing belonging to itself that is cognizable by us, to hold it together, but remains in existence owing to a force that is introduced into it. Now this power or force both is, and is called, nourishment. But it is not the same in all bodies that require aliment, but to each of them has been assigned a food adapted to its condition by Him who governs Nature. Some animals feed on roots which they dig up. Of others grass is the food, of others different kinds of flesh, but for man above all things bread; and, in order to continue and preserve the moisture of his body, drink, not simply water, but water frequently sweetened with wine, to join forces with our internal heat. He, therefore, who thinks of these things, thinks by implication of the particular bulk of our body. For those things by being within me became my blood and flesh, the corresponding nutriment by its power of adaptation being changed into the form of my body. With these distinctions we must return to the consideration of the question before us. The question was, how can that one Body of Christ vivify the whole of mankind, all, that is, in whomsoever there is Faith, and yet, though divided amongst all, be itself not diminished? Perhaps, then, we are now not far from the probable explanation. If the subsistence of every body depends on nourishment, and this is eating and drinking, and in the case of our eating there is bread and in the case of our drinking water sweetened with wine, and if, as was explained at the beginning, the Word of God, Who is both God and the Word, coalesced with man’s nature, and when He came in a body such as ours did not innovate on man’s physical constitution so as to make it other than it was, but secured continuance for His own body by the customary and proper means, and controlled its subsistence by meat and drink, the former of which was bread,—just, then, as in the case of ourselves, as has been repeatedly said already, if a person sees bread he also, in a kind of way, looks on a human body, for by the bread being within it the bread becomes it, so also, in that other case, the body into which God entered, by partaking of the nourishment of bread, was, in a certain measure, the same with it; that nourishment, as we have said, changing itself into the nature of the body. For that which is peculiar to all flesh is acknowledged also in the case of that flesh, namely, that that Body too was maintained by bread; which Body also by the indwelling of God the Word was transmuted to the dignity of Godhead. Rightly, then, do we believe that now also the bread which is consecrated by the Word of God is changed into the Body of God the Word. For that Body was once, by implication, bread, but has been consecrated by the inhabitation of the Word that tabernacled in the flesh. Therefore, from the same cause as that by which the bread that was transformed in that Body was changed to a Divine potency, a similar result takes place now. For as in that case, too, the grace of the Word used to make holy the Body, the substance of which came of the bread, and in a manner was itself bread, so also in this case the bread, as says the Apostle, “is sanctified by the Word of God and prayer”; not that it advances by the process of eating to the stage of passing into the body of the Word, but it is at once changed into the body by means of the Word, as the Word itself said, “This is My Body.” Seeing, too, that all flesh is nourished by what is moist (for without this combination our earthly part would not continue to live), just as we support by food which is firm and solid the solid part of our body, in like manner we supplement the moist part from the kindred element; and this, when within us, by its faculty of being transmitted, is changed to blood, and especially if through the wine it receives the faculty of being transmuted into heat. Since, then, that God-containing flesh partook for its substance and support of this particular nourishment also, and since the God who was manifested infused Himself into perishable humanity for this purpose, viz. that by this communion with Deity mankind might at the same time be deified, for this end it is that, by dispensation of His grace, He disseminates Himself in every believer through that flesh, whose substance comes from bread and wine, blending Himself with the bodies of believers, to secure that, by this union with the immortal, man, too, may be a sharer in incorruption. He gives these gifts by virtue of the benediction through which He transelements the natural quality of these visible things to that immortal thing.
St Gregory of Nyssa in The Great Catechism 37 (XXXVII): the bread changed by the consecration is transmuted into the very body of Christ, just as bread in ordinary digestion becomes the body of the one who eats it — but here by the Word's power. A foundational Eastern articulation of Eucharistic transformation that does not borrow Aristotelian categories.
St Gregory of Nyssa, The Great Catechism XXXVII
FideOrth 4.13.1
An Exact Exposition of the Orthodox Faith (De Fide Orthodoxa) · Saint John of Damascus · Ante-Nicene Fathers (Roberts–Donaldson)
God Who is good and altogether good and more than good, Who is goodness throughout, by reason of the exceeding riches of His goodness did not suffer Himself, that is His nature, only to be good, with no other to participate therein, but because of this He made first the spiritual and heavenly powers: next the visible and sensible universe: next man with his spiritual and sentient nature. All things, therefore, which he made, share in His goodness in respect of their existence. For He Himself is existence to all, since all things that are, are in Him, not only because it was He that brought them out of nothing into being, but because His energy preserves and maintains all that He made: and in especial the living creatures. For both in that they exist and in that they enjoy life they share in His goodness. But in truth those of them that have reason have a still greater share in that, both because of what has been already said and also because of the very reason which they possess. For they are somehow more dearly akin to Him, even though He is incomparably higher than they.
St John of Damascus, summarizing the patristic consensus: the bread and wine are not types of the body and blood, but the deified body and blood of the Lord Himself. The Holy Spirit at the priest's invocation effects this — the same Spirit by whom the Word took flesh of the Theotokos.
St John of Damascus, Exact Exposition of the Orthodox Faith IV.13
ApConst 8.4.6
Constitutions of the Holy Apostles · The Apostolic Tradition (anonymous, compilation) · Ante-Nicene Fathers (Roberts–Donaldson)
XXXIV. Offer up your prayers in the morning, at the third hour, the sixth, the ninth, the evening, and at cock-crowing: in the morning, returning thanks that the Lord has sent you light, that He has brought you past the night, and brought on the day; at the third hour, because at that hour the Lord received the sentence of condemnation from Pilate; at the sixth, because at that hour He was crucified; at the ninth, because all things were in commotion at the crucifixion of the Lord, as trembling at the bold attempt of the impious Jews, and not bearing the injury offered to their Lord; in the evening, giving thanks that He has given you the night to rest from the daily labours; at cock-crowing, because that hour brings the good news of the coming on of the day for the operations proper for the light. But if it be not possible to go to the church on account of the unbelievers, thou, O bishop, shalt assemble them in a house, that a godly man may not enter into an assembly of the ungodly. For it is not the place that sanctifies the man, but the man the place. And if the ungodly possess the place, do thou avoid it, because it is profaned by them. For as holy priests sanctify a place, so do the profane ones defile it. If it be not possible to assemble either in the church or in a house, let every one by himself sing, and read, and pray, or two or three together. For “where two or three are gathered together in my name, there am I in the midst of them.” Let not one of the faithful pray with a catechumen, no, not in the house: for it is not reasonable that he who is admitted should be polluted with one not admitted. Let not one of the godly pray with an heretic, no, not in the house. For “what fellowship hath light with darkness?” Let Christians, whether men or women, who have connections with slaves, either leave them off, or let them be rejected.
From the anaphora preserved in Book VIII of the Apostolic Constitutions — the so-called Clementine liturgy — perhaps the earliest fully-recorded Eucharistic prayer in Christian literature, with the explicit epiclesis on the gifts that they may become "the body of Thy Christ, and the blood of Thy Christ."
Apostolic Constitutions VIII (the Clementine liturgy)
HomMt 79.1.9
Homilies on the Gospel of Matthew · Saint John Chrysostom · Ante-Nicene Fathers (Roberts–Donaldson)
And He gives thanks, to teach us how we ought to celebrate this sacrament, and to show that not unwillingly doth He come to the passion, and to teach us whatever we may suffer to bear it thankfully, thence also suggesting good hopes. For if the type was a deliverance from such bondage, how much more will the truth set free the world, and will He be delivered up for the benefit of our race. Wherefore, I would add, neither did He appoint the sacrament before this, but when henceforth the rites of the law were to cease. And thus the very chief of the feasts He brings to an end, removing them to another most awful table, and He saith, “Take, eat, This is my body, Which is broken for many.”
St John Chrysostom, expounding the institution narrative: "And He gives thanks, to teach us how we ought to celebrate this Mystery... and that we may not approach it with troubled minds, but with thanksgiving." Chrysostom roots the Eucharist in Christ's own action of blessing and giving thanks.
St John Chrysostom, Homilies on Matthew 82 (NPNF) on the institution of the Eucharist