This year marks the centennial of the publication of Evelyn Underhill's magnum opus Mysticism. It has held up remarkably well and Underhill's work remains the subject of much scholarly and popular interest. To honor the book's 100th birthday I have culled a few quotes on theosis (which she calls by its synonym deification) from chapter 10. Two striking things about this material are her use of metaphors drawn from alchemy to describe deification, and also the fact that almost all the mystics referred to are western Christians, proving that theosis is not some parochial notion limited to Eastern Orthodox monks.

The complete text of the book may be found here in an online version.

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The mystic, I think, would acquiesce in these [psychological] descriptions, so far as they go: but he would probably translate them into his own words and gloss them with an explanation which is beyond the power and province of psychology. He would say that his long-sought correspondence with Transcendental Reality, his union with God, has now been finally established: that his self, though intact, is wholly penetrated--as a sponge by the sea--by the Ocean of Life and Love to which he attained. "I live, yet not I but God in me". He is conscious that he is now at length cleansed of the last stains of separation, and has become, in a mysterious manner, "that which he beholds."

...In the image of the alchemists, the Fire of Love has done its work: the mystic Mercury of the Wise--that little hidden treasure, that scrap of reality within him--has utterly transmuted the salt and sulphur of his mind and his sense. Even the white stone of illumination, once so dearly cherished, he has resigned to the crucible. Now, the great work is accomplished, the last imperfection is gone, and he finds within himself the "Noble Tincture"--the gold of spiritual humanity.

We have said that the mystic of the impersonal type--the seeker of a Transcendent Absolute--tends to describe the consummation of his quest in the language of deification. The Unitive Life necessarily means for him, as for all who attain it, something which infinitely transcends the sum total of its symptoms: something which normal men cannot hope to understand. In it he declares that he "partakes directly of the Divine Nature", enjoys the fruition of reality. Since we "only behold that which we are", the doctrine of deification results naturally and logically from this claim.

...Whilst the more clear-sighted are careful to qualify it in a sense which excludes pantheistic interpretations, and rebuts the accusation that extreme mystics preach the annihilation of the self and regard themselves as co-equal with the Deity, they leave us in no doubt that it answers to a definite and normal experience of many souls who attain high levels of spiritual vitality. Its terms are chiefly used by those mystics by whom Reality is apprehended as a state or place rather than a Person: and who have adopted, in describing the earlier stages of their journey to God, such symbols as those of rebirth or transmutation.

...The first thing which emerges from these reports, and from the choice of symbols which we find in them, is that the great mystics are anxious above all things to establish and force on us the truth that by deification they intend no arrogant claim to identification with God, but as it were a transfusion of their selves by His Self: an entrance upon a new order of life, so high and so harmonious with Reality that it can only be called divine. Over and over again they assure us that personality is not lost, but made more real. "When," says St Augustine, "I shall cleave to Thee with all my being, then shall I in nothing have pain and labour; and my life shall be a real life, being wholly full of Thee". "My life shall be a real life" because it is "full of Thee". The achievement of reality, and deification, are then one and the same thing: necessarily so, since we know that only the divine is the real.

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.