This early Christian understanding of sanctification was an important discovery of the pietist movement, and especially the Methodists, who used it to develop their distinctive Protestant doctrine of entire sanctification (which teaches, in summary, that it is the Christian's goal, in principle possible to achieve, to live without any sin).
The crucial Orthodox assertion, that God is One, sets an absolute limit on the meaning of theosis - it is not possible for any created being to become, ontologically, God or, another god. Rather, by deification is meant that, God causes Man to be in creaturely form everything that God is: that is, in terms of knowledge of the truth, in wisdom and in holiness, in all the energies of God.
That the Trinity is this one God, is also an essential assertion underlying the Orthodox doctrine of theosis. Belief in the Trinity is the indispensable premise of the uniqueness of Jesus Christ: that He alone is the Son of God and the Son of Man, the only one who is ontologically both God and Man, having both natures, unmixed and distinct, in one person.
The proverbial expression of theosis, "The Son of God became man, that we might become God" (— Athanasius of Alexandria), presupposes the limitations made explicit by the doctrine of the Trinity. That is, it is as absurd to suppose that a creature may become the eternally self-existent God, as that God would be changed into a creature. But what would otherwise seem to be absurd, that sinful people may become holy as God is holy, this has been made possible through Jesus Christ, who is God incarnate. Substantially the same idea can be expressed in terms more familiar to the Western Church: through the knowledge of God in Jesus Christ, Man comes to know and experience what it means to be fully Man (the created image of God); through Man's communion with Jesus Christ God shares Himself with Man, in order to conform Man to all that God is in knowledge, righteousness and holiness.
Theosis also asserts the complete restoration of all men (and of the entire creation), in principle. This is built upon the understanding of the atonement put forward by Irenaeus, called "recapitulation". All of mankind is fully restored to the full potential of humanity because the Son of God took to Himself a human nature to be born of a woman, and takes to Himself also the sufferings due to sin (yet is not Himself a sinful man, and is God unchanged in His being). In Christ, the two natures of God and Man are not two persons but one; thus, a union is effected in Christ, between all of mankind and God. So, the holy God and sinful Man are reconciled in principle, in the one sinless man, Jesus Christ. (See Jesus's prayer as recorded in John 17.) This reconciliation is made actual through the struggle to conform to the image of Christ.
For many fathers, theosis goes beyond simply restoring man to his state before the Fall of Adam and Eve. These fathers teach that because Christ united the human and divine natures in his person, it is now possible for man to experience closer fellowship with God than Adam and Eve initially experienced in the Garden of Eden, and that man can become more like God than Adam and Eve were at that time. Some Orthodox theologians go so far as to say that Jesus would have become incarnate for this reason alone, even if Adam and Eve had never sinned.
The course of man's spiritual journey toward perfection and a holy life is set then, along the path established by what God has done in Christ: man must unite will, thought and action to God's will, His thoughts and His actions. Man must fashion his life to be a mirror, a true likeness of God. More than that, since God and Man are more than a similarity in Christ but rather a true union, the life of Man is more than mere imitation and is rather a union with the life of God Himself: so that, the man who is working out his salvation, is God working within the man both to will and to do that which pleases God. Gregory Palamas affirmed the possibility of man's union with God in His energies, while also affirming that because of God's transcendance and utter other-ness, it is impossible for any man or other creature to know or to be united with God's essence.
The journey towards theosis includes many things. Living in the community of the church and partaking regularly of the sacraments, and especially the Eucharist, is taken for granted. Also important is cultivating "prayer of the heart", and prayer that never ceases, as Paul exhorts the Thessalonians to do. This unceasing prayer of the heart is a dominant theme in the writings of the Fathers, especially in those collected in the Philokalia.
Orthodox Christianity's Theosis is thus usually regarded as an orthodox, Christian understanding of salvation from sin, in both the East and the West (although it is not everywhere regarded as the most helpful or clear terminology) — but such approval only applies where the term is used in the context of Trinitarian Christianity. Otherwise, theosis, the effort to put oneself in the place of God, is the kernel expression of all sin and false religion, and in fact Satan's own sin and the spirit of Antichrist, according to the Trinitarian Christian view.
See also: Desert Fathers, Maximus the Confessor, Monasticism, Philokalia
The doctrine of theosis or deification in The Church of Jesus Christ of Latter-day Saints differs significantly from the theosis of Orthodox Christianity. In Mormonism it is usually referred to as exaltation or eternal life. While the primary focus of Mormonism is on the atonement of Jesus Christ, the reason for the atonement is exaltation which goes beyond mere salvation. All men will be saved from sin and death, but only those who are sufficiently obedient and accept the atonement of Jesus Christ before the judgment will be exalted.
In Moses 1:39 God tells Moses, "this is my work and my glory—to bring to pass the immortality and eternal life of man". In that chapter God shows Moses a vision depicting some of God's vast creations including a vast number of worlds created for other people—a sampling of what God created in the past and what he will continue to do forever. Each world was prepared and peopled by God for the purpose of bringing to pass the immortality and eternal life of humankind. By immortality is meant personal resurrection so that each individual can continue to enjoy a perfect, physical body forever. By eternal life is meant becoming like God both in terms of holiness or godliness and in glory. It is commonly believed by members of the Church that, like God, an exalted human being is empowered with the privilege to create worlds and people in an endless process of exalting humankind.
Of all the Mormon doctrines including polygamy, critics generally deem this doctrine the most offensive or even blasphemous. Some Mormons argue that even assuming mainstream Christianity's definition of God's omnipotence and omnibenevolence, not only can God exalt mortal man, but God must do so. The argument is that if God is all-powerful, then God is capable of exalting man, and if God is all-good, then God should or must exalt man. They also point to comments by Christ and Psalmists among others that refer to the Divine nature and potential of humans as children of God. Some Mormons also suggest that discussions of theosis by early Church fathers show an early belief in the Mormon concept of deification, although they disagree with much of the other theology of the same Church fathers.
Alternative spellings: Theiosis, TheopoiesisTheosis in Orthodox Christianity
Deification in Mormonism