Among the Eastern Orthodox, theosis is considered a central element Christianity. In the West, until recently, theosis was generally ignored but, when not, was often considered heretical, and it’s easy to see why. Many in the East speak of “deification” or “divinization” of the Christian, as though we become somehow co-equal with God.
That’s not really the teaching, but it’s easy to understand the confusion when such terms are tossed about — and I’ll not be using those terms because that is not what I mean by theosis.
More recently, in the West theosis is becoming a respectable term, thanks in substantial part to the work of Michael J. Gorman. Some Eastern writers may have used overwrought language, but the NT certainly teaches a doctrine of the unity of the saved with God and Jesus. Actually, it’s all over the pages of scripture once you start looking for it. In fact, the Orthodox are right to point out that it’s an important NT concept.
Classic NT texts
Famously, Jesus said,
(Joh 17:20-23 ESV) “I do not ask for these only, but also for those who will believe in me through their word, 21 that they may all be one, just as you, Father, are in me, and I in you, that they also may be in us, so that the world may believe that you have sent me. 22 The glory that 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 become perfectly one, so that the world may know that you sent me and loved them even as you loved me.”
Notice, that Jesus speaks in terms of X being “in” Y. Jesus is in God. God is in Jesus. He prays that his disciples be in both God and Jesus, and speaks of himself being in his disciples.
This mutual in-ness results in unity or being one. Hence, God and Jesus are one. He prays that the disciples be one with each other (see also John 17:11).
However, Jesus does not pray that the disciples be one with God and Jesus in the same sense in which Jesus and God are one with each other. Rather, both in 17:11 and 17:23, the point is that they be one with each other. However, there is a mutual in-ness between the disciples and Jesus.
In short, Jesus doesn’t quite suggest that the disciples become a part of the Trinity. He is not praying for one-ness of this kind. And yet there is a real teaching of mutual indwelling.
Lenski sees the profundity of the statement better than most —
In v. 11 the disciples are to be one “even as we” (Jesus and the Father); in v. 21 all believers are to be one “even as thou art in me, and I in thee,” i.e., as we (Jesus and the Father) are one. But now we see how these two are one, namely by the ineffable divine interpenetration. And now Jesus adds the second ἵνα [hina] as appositional to the first. It defines how all the believers are to be one, how their oneness is to resemble that of the divine Persons, namely thus: “that also they be in us.” All believers being in the Father and the Son, they will certainly be one.
This will not be a mere human oneness (national, racial, political, in a society, or the like). It will bear the divine stamp: a. oneness in the true God, in actual spiritual union with him. This is why so high a model and pattern is set for our oneness. Here, too, we see that our oneness is not merely placed beside the oneness of the divine Persons as though all that exists between them is a likeness. The two are vitally connected; this is why they have the resemblance of which Jesus speaks.
We believers can be one with each other only by each of us and all of us being one with the Father and Jesus. Union with God and with Christ makes us a unit in ourselves. Jesus here omits the other side, namely that as the believers are in God and his Son, so these are also in us (14:23, add v. 16).
R. C. H. Lenski, The Interpretation of St. John’s Gospel, (Minneapolis, MN: Augsburg Publishing House, 1961), 1156–1157.
The divine nature
(2Pe 1:3-4 ESV) His divine power has granted to us all things that pertain to life and godliness, through the knowledge of him who called us to his own glory and excellence, 4 by which he has granted to us his precious and very great promises, so that through them you may become partakers of the divine nature, having escaped from the corruption that is in the world because of sinful desire.
Again, Lenski realizes the depth of what Peter teaches better than most —
For this “divine nature” is not the substantia but the qualitas; it is more than the imitatio, it is rather the imago Dei. As a foreigner is naturalized, so we are fully transplanted into God’s kingdom and are naturalized in it so that what is in that kingdom is properly ours. We are to be children and sons of God (John 1:12), begotten again, not of corruptible seed, but of incorruptible, through the Word of God which lives and abides forever (1 Pet. 1:23). Ours is the restored divine image, righteousness and holiness (Eph. 4:24) plus knowledge (epignosis, Col. 3:10). The two former are divine attributes. When they are restored in us they do not deify us; yet they are derived from God and make us κοινωνοί [participants] of divine nature. Here belong all those passages that speak of the unio mystica such as Gal. 2:20: “Christ lives in me”; Phil. 1:21: “For me to live is Christ”; John 15:4, 5, “you in me, and I in you”; 14:23 and 1 John 2:24, also Rev. 3:20, which describe the koinonia.
R. C. H. Lenski, The Interpretation of the Epistles of St. Peter, St. John and St. Jude, (Minneapolis, MN: Augsburg Publishing House, 1966), 262.
It’s a theme coursing through John, Peter, and Paul: when we are saved, the eschatological unity we will have after the Second Coming is immediately received although not yet fully realized. It’s part of the doctrine called “inaugurated eschatology,” that is, that Rev 21 and 22 are already true of the church in some very real sense, although not fully so. We become engaged to Jesus at baptism — a part of his bride — but the marriage comes when Jesus returns.
(1Co 13:9-12 ESV) 9 For we know in part and we prophesy in part, 10 but when the perfect comes, the partial will pass away. 11 When I was a child, I spoke like a child, I thought like a child, I reasoned like a child. When I became a man, I gave up childish ways. 12 For now we see in a mirror dimly, but then face to face. Now I know in part; then I shall know fully, even as I have been fully known.
We should not limit what might happen in this age. There is no limit as to how united the church might become within itself and with Jesus and God in this age compared to the next. The future promises also speak of today. The promises for today also speak of the future.