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.