Theosis (deification) in the Fathers

The doctrine that the goal of the Christian life is union with God by participation in the divine life: that the Word became flesh, as St Athanasius wrote, "that we might be made God" (Inc 54). The Greek Fathers articulate this as participation in the divine nature (2 Pet 1:4) through the energies of the Holy Spirit, never confusing the creature with the uncreated essence.

2Pet 1:1-11
2 Peter · Holy Apostle Peter · World English Bible (NT)
¹ Simon Peter, a servant and apostle of Jesus Christ, to those who have obtained a like precious faith with us in the righteousness of our God and Savior, Jesus Christ: ² Grace to you and peace be multiplied in the knowledge of God and of Jesus our Lord, ³ seeing that his divine power has granted to us all things that pertain to life and godliness, through the knowledge of him who called us by his own glory and virtue, ⁴ by which he has granted to us his precious and exceedingly great promises; that through these you may become partakers of the divine nature, having escaped from the corruption that is in the world by lust. ⁵ Yes, and for this very cause adding on your part all diligence, in your faith supply moral excellence; and in moral excellence, knowledge; ⁶ and in knowledge, self-control; and in self-control, perseverance; and in perseverance, godliness; ⁷ and in godliness, brotherly affection; and in brotherly affection, love. ⁸ For if these things are yours and abound, they make you to not be idle or unfruitful in the knowledge of our Lord Jesus Christ. ⁹ For he who lacks these things is blind, seeing only what is near, having forgotten the cleansing from his old sins. ¹⁰ Therefore, brothers, be more diligent to make your calling and election sure. For if you do these things, you will never stumble. ¹¹ For thus you will be richly supplied with the entrance into the eternal Kingdom of our Lord and Savior, Jesus Christ.
"Through these he has granted to us his precious and exceedingly great promises; that through these you may become partakers of the divine nature, having escaped from the corruption that is in the world by lust." The scriptural anchor for the patristic doctrine of theosis, and citing 2 Peter 1:4 is virtually universal in the Greek Fathers.
2 Peter 1:4 (the foundational scriptural text)
Jn 17:20-26
John · Holy Apostle and Evangelist John the Theologian · World English Bible (NT)
²⁰ “Not for these only do I pray, but for those also who will believe in me through their word, ²¹ that they may all be one; even as you, Father, are in me, and I in you, that they also may be one in us; that the world may believe that you sent me. ²² The glory which you have given me, I have given to them, that they may be one, even as we are one, ²³ I in them, and you in me, that they may be perfected into one, that the world may know that you sent me and loved them, even as you loved me. ²⁴ Father, I desire that they also whom you have given me be with me where I am, that they may see my glory which you have given me, for you loved me before the foundation of the world. ²⁵ Righteous Father, the world hasn’t known you, but I knew you; and these knew that you sent me. ²⁶ I made known to them your name, and will make it known; that the love with which you loved me may be in them, and I in them.”
Christ's High Priestly prayer: "that they may all be one; even as you, Father, are in me, and I in you, that they also may be one in us..." Read by the Fathers as the dominical prayer that promises participation in the divine life of the Trinity.
Jn 17:20-26; cited by St Athanasius, St Maximus, and the Cappadocians
AH 5.1.1
Against Heresies (Adversus Haereses) · Saint Irenaeus of Lyons · Ante-Nicene Fathers (Roberts–Donaldson)
In the four preceding books, my very dear friend, which I put forth to thee, all the heretics have been exposed, and their doctrines brought to light, and these men refuted who have devised irreligious opinions. [I have accomplished this by adducing] something from the doctrine peculiar to each of these men, which they have left in their writings, as well as by using arguments of a more general nature, and applicable to them all. Then I have pointed out the truth, and shown the preaching of the Church, which the prophets proclaimed (as I have already demonstrated), but which Christ brought to perfection, and the apostles have handed down, from whom the Church, receiving [these truths], and throughout all the world alone preserving them in their integrity (bene), has transmitted them to her sons. Then also—having disposed of all questions which the heretics propose to us, and having explained the doctrine of the apostles, and clearly set forth many of those things which were said and done by the Lord in parables—I shall endeavour, in this the fifth book of the entire work which treats of the exposure and refutation of knowledge falsely so called, to exhibit proofs from the rest of the Lord’s doctrine and the apostolical epistles: [thus] complying with thy demand, as thou didst request of me (since indeed I have been assigned a place in the ministry of the word); and, labouring by every means in my power to furnish thee with large assistance against the contradictions of the heretics, as also to reclaim the wanderers and convert them to the Church of God, to confirm at the same time the minds of the neophytes, that they may preserve stedfast the faith which they have received, guarded by the Church in its integrity, in order that they be in no way perverted by those who endeavour to teach them false doctrines, and lead them away from the truth. It will be incumbent upon thee, however, and all who may happen to read this writing, to peruse with great attention what I have already said, that thou mayest obtain a knowledge of the subjects against which I am contending. For it is thus that thou wilt both controvert them in a legitimate manner, and wilt be prepared to receive the proofs brought forward against them, casting away their doctrines as filth by means of the celestial faith; but following the only true and stedfast Teacher, the Word of God, our Lord Jesus Christ, who did, through His transcendent love, become what we are, that He might bring us to be even what He is Himself.
St Irenaeus (c. 180), at the opening of Against Heresies V: the Word of God "did, through His transcendent love, become what we are, that He might bring us to be even what He is Himself." Possibly the earliest sustained articulation of the great exchange that grounds theosis.
St Irenaeus, Against Heresies V, opening of Book V
AH 3.19.1
Against Heresies (Adversus Haereses) · Saint Irenaeus of Lyons · Ante-Nicene Fathers (Roberts–Donaldson)
1. As it has been clearly demonstrated that the Word, who existed in the beginning with God, by whom all things were made, who was also always present with mankind, was in these last days, according to the time appointed by the Father, united to His own workmanship, inasmuch as He became a man liable to suffering, [it follows] that every objection is set aside of those who say, “If our Lord was born at that time, Christ had therefore no previous existence.” For I have shown that the Son of God did not then begin to exist, being with the Father from the beginning; but when He became incarnate, and was made man, He commenced afresh the long line of human beings, and furnished us, in a brief, comprehensive manner, with salvation; so that what we had lost in Adam—namely, to be according to the image and likeness of God—that we might recover in Christ Jesus.
St Irenaeus: "Our Lord Jesus Christ, the Word of God, of His boundless love, became what we are, that He might make us to be what He is Himself." The great exchange formula, God became human so that human beings might be deified, recurs in Athanasius, Gregory of Nazianzus, and Maximus.
St Irenaeus, Against Heresies III.19.1
Inc 54.1
On the Incarnation of the Word · Saint Athanasius the Great of Alexandria · Ante-Nicene Fathers (Roberts–Donaldson)
§54. The Word Incarnate, as is the case with the Invisible God, is known to us by His works. By them we recognise His deifying mission. Let us be content to enumerate a few of them, leaving their dazzling plentitude to him who will behold.
St Athanasius the Great: "He was made man that we might be made God; and He manifested Himself by a body that we might receive the idea of the unseen Father; and He endured the insolence of men that we might inherit immortality." The foundational Athanasian dictum on theosis, the goal of the Incarnation.
St Athanasius, On the Incarnation of the Word 54
CA 1.13.5
Discourses Against the Arians (Orationes contra Arianos) · Saint Athanasius the Great of Alexandria · Ante-Nicene Fathers (Roberts–Donaldson)
(5.) If moreover, baffled so far, they should still violently insist that the language is that of comparison, and that comparison in consequence implies oneness of kind, so that the Son is of the nature of Angels, they will in the first place incur the disgrace of rivalling and repeating what Valentinus held, and Carpocrates, and those other heretics, of whom the former said that the Angels were one in kind with the Christ, and Carpocrates that Angels are framers of the world. Perchance it is under the instruction of these masters that they compare the Word of God with the Angels.
St Athanasius in the Discourses Against the Arians: if the Son were merely a creature, He could not deify us. Only because the Son shares the Father's nature can He communicate it. Theosis is therefore an inseparable consequence of the Nicene homoousion.
St Athanasius, Discourses Against the Arians I.39 and II.70
Spir 9.1.4
On the Holy Spirit (De Spiritu Sancto) · Saint Basil the Great · Ante-Nicene Fathers (Roberts–Donaldson)
23. Now the Spirit is not brought into intimate association with the soul by local approximation. How indeed could there be a corporeal approach to the incorporeal? This association results from the withdrawal of the passions which, coming afterwards gradually on the soul from its friendship to the flesh, have alienated it from its close relationship with God. Only then after a man is purified from the shame whose stain he took through his wickedness, and has come back again to his natural beauty, and as it were cleaning the Royal Image and restoring its ancient form, only thus is it possible for him to draw near to the Paraclete. And He, like the sun, will by the aid of thy purified eye show thee in Himself the image of the invisible, and in the blessed spectacle of the image thou shalt behold the unspeakable beauty of the archetype. Through His aid hearts are lifted up, the weak are held by the hand, and they who are advancing are brought to perfection. Shining upon those that are cleansed from every spot, He makes them spiritual by fellowship with Himself. Just as when a sunbeam falls on bright and transparent bodies, they themselves become brilliant too, and shed forth a fresh brightness from themselves, so souls wherein the Spirit dwells, illuminated by the Spirit, themselves become spiritual, and send forth their grace to others. Hence comes foreknowledge of the future, understanding of mysteries, apprehension of what is hidden, distribution of good gifts, the heavenly citizenship, a place in the chorus of angels, joy without end, abiding in God, the being made like to God, and, highest of all, the being made God. Such, then, to instance a few out of many, are the conceptions concerning the Holy Spirit, which we have been taught to hold concerning His greatness, His dignity, and His operations, by the oracles of the Spirit themselves.
St Basil the Great in On the Holy Spirit IX §23: "Now the Spirit is not brought into intimate association with the soul by local approximation… we are made like God, and (highest of all the achievements) we are made God." The Spirit's deifying action grounds the doctrine of the Spirit's divinity against the Pneumatomachoi.
St Basil the Great, On the Holy Spirit IX §23
Or 1.1.8
Select Orations · Saint Gregory the Theologian · Ante-Nicene Fathers (Roberts–Donaldson)
V. Let us become like Christ, since Christ became like us. Let us become God’s for His sake, since He for ours became Man. He assumed the worse that He might give us the better; He became poor that we through His poverty might be rich; He took upon Him the form of a servant that we might receive back our liberty; He came down that we might be exalted; He was tempted that we might conquer; He was dishonoured that He might glorify us; He died that He might save us; He ascended that He might draw to Himself us, who were lying low in the Fall of sin. Let us give all, offer all, to Him Who gave Himself a Ransom and a Reconciliation for us. But one can give nothing like oneself, understanding the Mystery, and becoming for His sake all that He became for ours.
St Gregory the Theologian in his First Oration (on Pascha): "Let us become like Christ, since Christ became like us. Let us become God's for His sake, since He for ours became Man." The paschal articulation of theosis, and one of the most-quoted Greek patristic dicta on deification.
St Gregory of Nazianzus, Oration I (On Pascha) §5
FideOrth 4.13.5
An Exact Exposition of the Orthodox Faith (De Fide Orthodoxa) · Saint John of Damascus · Ante-Nicene Fathers (Roberts–Donaldson)
If then the Word of God is quick and energising, and the Lord did all that He willed; if He said, Let there be light and there was light, let there be a firmament and there was a firmament; if the heavens were established by the Word of the Lord and all the host of them by the breath of His mouth; if the heaven and the earth, water and fire and air and the whole glory of these, and, in sooth, this most noble creature, man, were perfected by the Word of the Lord; if God the Word of His own will became man and the pure and undefiled blood of the holy and ever-virginal One made His flesh without the aid of seed, can He not then make the bread His body and the wine and water His blood? He said in the beginning, Let the earth bring forth grass, and even until this present day, when the rain comes it brings forth its proper fruits, urged on and strengthened by the divine command. God said, This is My body, and This is My blood, and this do ye in remembrance of Me. And so it is at His omnipotent command until He come: for it was in this sense that He said until He come: and the overshadowing power of the Holy Spirit becomes through the invocation the rain to this new tillage. For just as God made all that He made by the energy of the Holy Spirit, so also now the energy of the Spirit performs those things that are supernatural and which it is not possible to comprehend unless by faith alone. How shall this be, said the holy Virgin, seeing I know not a man? And the archangel Gabriel answered her: The Holy Spirit shall come upon thee, and the power of the Highest shall overshadow thee. And now you ask, how the bread became Christ’s body and the wine and water Christ’s blood. And I say unto thee, “The Holy Spirit is present and does those things which surpass reason and thought.”
St John of Damascus on the deifying action of the Eucharist: "It is called participation because through it we participate in Jesus' divinity. It is called communion, and truly is, because of our communion through it with Christ." Theosis is concretely received in the holy mysteries.
St John of Damascus, Exact Exposition IV.13