Theosis should not be understood as a philosophical theory but is grounded in scripture, tradition and the writings of the early Christian writers.
The following scripture passages summarizes Theosis:
Psalm 82.6: I said, "You are gods"; you are all sons of the most high.
2 Peter 1:3–4: God’s “divine power has given to us all things that pertain to life and godliness” through the knowledge of God, who called us by His own glory and goodness. Through these things, He has given us His great promises so that we “may be partakers of the divine nature, having escaped the corruption that is in the world through lust.”
Romans 12:1–2: We are to present our bodies as a “living sacrifice,” doing so as part of our spiritual worship. And we are to “be transformed” by the renewing of our minds into the likeness of God.
1 Corinthians 3:16; 6:17: We are reminded that we are God’s “temple” and that “he who is joined to the Lord is one spirit with Him”—union with God.
Galatians 2:20: “It is no longer I who live, but Christ who lives in me.”
Philippians 1:21: “For me, to live is Christ.”
Colossians 3:3: We have “died” and our lives are “hidden with Christ in God”—total participation in Christ.
1 Thessalonians 5:23: May God “sanctify you completely”—complete conformity to the image and likeness of God.
2 Thessalonians 2:14: We were called by God “for the obtaining of the glory of our Lord Jesus Christ.”
1 John 4:17: “Because as He is, so are we in this world”—the possibility of deification, total participation in Christ this side of eternity.
John 17:22: In His high priestly prayer, Jesus says that He has given us the glory that the Father gave Him.
Revelation 21:7: At the beginning of the eschaton, Christ says of each of us, “I will be his God and he shall be My son.”
1 John 3:2: “We know that when He is revealed, we shall be like Him, for we shall see Him as He is.”
Philippians 3:21: Christ will “transform our lowly body that it may be conformed to His glorious body.”
The early Christian fathers bore witness to Theosis. More often it was the Alexandrian Fathers who preached and defended Theosis as being the aim of the Christian life. It was because of Christ's incarnation, death and resurrection that we as humans are able to achieve this union in God through Christ. Christ show's us what it is to be God in the way he lived and died as a human being. The following are a few quotations from the Fathers writing on Theosis:
St. Athanasius: "God became man so that men might become gods".
St. Gregory of Nazianzuz (who's liturgy is prayed in the Coptic Church): "Man has been ordered to become God"...and..."For that which He has not assumed He has not healed; but what is united with God is also being saved".
St. Basil the Great (who's liturgy is prayed in the Coptic Church and Byzantine Church): “From the Holy Spirit is the likeness of God, and the highest thing to be desired, to become God.”
Origen noted that the spirit “is deified by that which it contemplates.”
St. Cyril of Alexandria commented that we are all called to take part in divinity, becoming the likeness of Christ and the image of the Father by “participation.”
St. Irenaeus noted, “If the Word is made man, it is that man might become gods.”
Theosis is a truly catholic understanding of the goal of our relationship with God in Christ. It is through Theosis that we can truly become human. St. Irenaeus reminds us that, "The glory of God is a living (alive) human being". In God we live and become life for others. The reason for our existence is made whole in God's existence (taking on human flesh) and the reason the Creator united with us was to complete, in its fullness, the bestowing of His grace on us because he gives abundantly. The best of gifts that God could have given us was His own self and it is because of this gift we are able to unite with Him. This is the first steps in achieving our potential in Christ!
Of the centrality of theosis and of theosis in Orthodox missiology, His Eminence Metropolitan Geevarghese Mar Osthathiose (Indian Orthodox) writes:
“Salvation is more than liberation of humanity or humanization, but divinization of the humanity and the cosmos. Finite salvation of the created is not infinite liberation, that is the infinite divinization as humanity is not created for one another alone, but for the Creator as well.
The aim of salvation is not restitution of the unfallen state and Adam and Eve, but elevation to the status and fulness of the Second Adam, which is called Christification or Trinitification.
The first Adam fell when tempted, but the Second Adam did not fall. Therefore our aim is not just humanization, but theosis. It is for this theosis that the Incarnation took place as "good news of a great joy which will come to all the people" (Lk 2:10)...
Ulltimately all the differences and separations between human beings will be dissolved in a mutual sharing of beingness (perichoresis) where 'thine and 'mine' are different in the case of property, purpose or will, but different only in different personal and group identities with full openness to penetrate each other...
The aim of mission can't be anything less than the deification, unification and reconciliation of all churches and the whole world into the unity of the measure and the stature of the fullness of Christ. It is not humanization or socialization but divinization, which is social transformation in the model of Holy Trinity, which may be called Trinification. The aim of mission is not only Theosis but along with it the establishment of the Kingdom of God.”
+ Metropolitan Geevarghese Mar Osthathiose . “Sharing God and a Sharing World” (India: ISPCK & CSS, 1995), 150-152.