I have made no bones about the fact that I think there is enormous congruity between what the East calls theosis and what the Lutherans of the 16th and 17th centuries termed mystical union. The loss of that whole mystical union way of thinking has been a sad loss, in my estimation, for the Lutheran Church, and it has disfigured us. The exclusive description of justification in forensic terms has been reduced to the imparting of information: God declares you righteous for Christ's sake. That's indeed the truth, but it is not all of the truth.
Let me put this way: it seems to me that we Lutherans have come over the years to speak of forgiveness almost as a thing, a substance, something along the lines of Rome's created grace, but even worse, because for Lutherans "forgiveness of sins" has come to be thought of as a mere "get out of hell free" card. Holding it, we think it excuses and indeed wipes out the consequences of lives of impenitence, ignoring the fact that our Confessions are utterly clear that saving faith exists ONLY in penitence. But the whole point of forgiveness is that it is NOT a thing, a substance, or a get out of hell free card. Forgiveness is the Blessed Trinity Himself coming to us in grace, so that His coming to us may be the advent of life and not of the judgment and death we so richly deserve. Forgiveness is given so that "life and salvation" may come with it, that is, that we may live in koinonia with God through Christ, receiving the divine life and being transfigured by
that life. "But we all, with unveiled face, beholding as in a glass the glory of the Lord are changed into the same image from glory to glory, even as by the Spirit of the Lord." (2 Cor. 3:18)
So God declares us righteous and then He goes on to make us righteous. Obviously salvation cannot depend upon His making us righteous, since that will never be completed in this age, so long as we remain in the sinful flesh. But both go inevitably together. As Luther explicated it time and again: "grace" and "the gift in grace." He took grace for the pardoning verdict of God upon a human life, but with that verdict came the Holy Spirit himself, the Gift in grace, who renews us precisely by uniting us to the Son and through the Son to the Father. "That which we have seen and heard, declare we unto you, that ye also may have fellowship with us, and truly our fellowship is with the Father and with his Son Jesus Christ." (1 John 1:3)
And the Lutheran Confessions explicitly speak of some form of cooperation between the new self and the Holy Spirit, no matter how weak it may be. The Large Catechism clearly speaks in "process" language when it says "In baptism we are given the grace, Spirit, and strength to suppress the old creature so that the new may come forth and grow strong" (V:76) and "Now, when we enter Christ's kingdom, this corruption must daily decrease so that the longer we live the more gentle, patient, and meek we become, and the more we break away from greed, hatred, envy and pride." (V:67) The Formula expresses the same reality in these words: "It has been sufficiently explained above how God makes willing people out of rebellious and unwilling people through the drawing power of the Holy Spirit, and how after this conversion of the human being the reborn will is not idle in the daily practice of repentance but cooperates in all works of the Holy Spirit that He accomplishes through us." SD FC II:88
When the Orthodox speak of "salvation" meaning by that BOTH the merciful forgiveness of God AND the interior renewal of the person (in which the person cooperates with the Holy Spirit), I fail to see how this in and of itself contradicts what our Confessions expressly describe. Salvation is pure gift, indeed. But it is LIVING gift, and it enlivens, and so embraces the new man in his cooperation with the Holy Spirit, and a mark of the new man's cooperation with the Spirit is that he rightly attributes the entirety of his salvation to the mercy and the grace of God, and not to himself.
St. Mark the Ascetic once expressed it like this: "Christ is Master by virtue of His own essence and Master by virtue of His incarnate life. For He creates man from nothing, and through His own Blood redeems him when dead in sin; and to those who believe in Him He has given His grace. When Scripture says, 'He will reward every man according to his works' (Matt 16:27), do not imagine that works in themselves merit either hell or the kingdom. On the contrary, Christ rewards each man according to whether his works are done with faith or without faith in Himself; and He is not a dealer bound by contract, but God our Creator and Redeemer." This is from his treatise in the Philokalia titled "On Those Who Think They Are Made Righteous by Works."