“Theosis” and “deification” are culpably ambiguous terms. On the other hand, using one of them in the title did get you to click on the link, did it not?
There are unobjectionable (as well as objectionable) ways of construing the concept behind the term, and there were senses in which the Reformers did so, as Steven Wedgeworth demonstrated here. I’ve also touched on it briefly here.
The most frequently quoted aphorism in connection with this topic is Athanasius, On the Incarnation of the Word 54:
As, then, if a man should wish to see God, Who is invisible by nature and not seen at all, he may know and apprehend Him from His works: so let him who fails to see Christ with his understanding, at least apprehend Him by the works of His body, and test whether they be human works or God’s works. 2. And if they be human, let him scoff; but if they are not human, but of God, let him recognise it, and not laugh at what is no matter for scoffing; but rather let him marvel that by so ordinary a means things divine have been manifested to us, and that by death immortality has reached to all, and that by the Word becoming man, the universal Providence has been known, and its Giver and Artificer the very Word of God. 3. For 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.
Gregory of Nazianzus says something similar in Oration 29.19:
For He Whom you now treat with contempt was once above you. He Who is now Man was once the Uncompounded. What He was He continued to be; what He was not He took to Himself. In the beginning He was, uncaused; for what is the Cause of God? But afterwards for a cause He was born. And that cause was that you might be saved, who insult Him and despise His Godhead, because of this, that He took upon Him your denser nature, having converse with Flesh by means of Mind. While His inferior Nature, the Humanity, became God, because it was united to God, and became One Person because the Higher Nature prevailed in order that I too might be made God so far as He is made Man.
In chapter 2 (De fide, “On [the] faith”) of the first edition of the Institutes of the Christian Religion (1536), John Calvin riffs on this idea in connection with Christ’s mediatorship and our adoption as sons of God. Calvin of course does not think that our substance is transformed into the uncompounded divine nature (neither did Athanasius and Gregory), which would be a functional denial of the Trinity (and thus heretical) as well as the destruction of the created integrity of human nature; but he does think that through union with the incarnate Christ by faith we share in his blessings, receiving by grace what Christ has by nature.
It was no common thing that had to be performed by the Mediator, viz., to make sons of God of the sons of men, heirs of the heavenly kingdom from heirs of hell. Who was able to do this, unless the Son of God became a son of man, and took on what was ours in such a way that he transferred to us what was his, and made that which was his by nature ours by grace? This therefore is our hope that we are sons of God: because the natural Son of God fashioned for himself a body from our body, flesh from our flesh, bones from our bones, in order that he might be the same as us. That which was proper to us he willed to belong to himself, in order that that which was proper to him might belong to us, and thus he himself might be both Son of God and Son of Man in common with us. This is our hope that the inheritance of the heavenly kingdom is ours, because the only-begotten Son of God, whose inheritance was unimpaired, has adopted us as his brothers: but if brothers, then partakers of the inheritance as well.1
Thus as Christ is Son of God and Son of Man, so he takes sons of men and makes them sons of God, graciously granting them the inheritance of the saints in light: life, immortality, incorruption, holiness, blessedness, and the fruition of the presence of God forevermore.
