Lutheran theology rests mainly on the concept of justification by faith, with "justification" understood in juridical terms and "faith" conceptualized more as an act of the will rather than an experience of the mystical heart. Theosis, the notion of intimate union with God so characteristic of Eastern Christianity, is generally not the first thing that pops into one's head when hearing "A Mighty Fortress is Our God".

However, Lutheran scholars in Finland have over the past few decades reevaluated their theological corpus. Aided by an ongoing dialog with the Russian Orthodox Church--Finland was ruled by Russia for over a century and has its own indigenous Orthodox church--Lutheran theologians such as Tuomo Manermaa and Simo Puera have taken a fresh look at the former Augustinian monk and have uncovered consistent references to theosis in his voluminous writings. Their work is summarized by Jonathan Linman in an essay appearing in Partakers of the Divine Nature: the history and development of deification in the Christian traditions (eds Michael J Christensen and Jeffery A Wittung, Baker Academic 2007). The article (on pp 189-199) contains several quotes from Luther's writings which I reproduce below.

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Just as the word of God became flesh, so it is certainly also necessary that the flesh may become word. In other words: God becomes man so that man may become God. Thus power becomes powerless so that weakness may become powerful. [1514 Christmas sermon]

We should not doubt at all that whenever one is being baptized the heavens are assuredly open and the entire Trinity is present and through its own presence sanctifies and blesses the person being baptized. [The Freedom of a Christian]

Christ appointed these two forms of bread and wine, rather than any other, as a further indication of the very union and fellowship which is in this sacrament. For there is no more intimate, deep, and indivisible union than the union of the food with him who is fed. For the food enters into and is assimilated by his very nature and becomes one substance with the person who is fed. Other unions, achieved by such things as nails, glue, cords, and the like, do not make one indivisible substance of the objects joined together. Thus in the sacrament we become united with Christ, and are made one body with all the saints, so that Christ cares for us and acts on our behalf.