Theosis, meaning deification or divinization, is the process of man becoming holy and being united with God, beginning in this life and later consummated in the resurrection. Theosis is the content of salvation from sin, is premised upon apostolic and early Christian understanding of the life of faith, and is conceptually foundational in both the east and the west.
Alternative spellings: Theiosis, Theopoiesis
The statement by St. Athanasius of Alexandria, "The Son of God became man, that we might become God", indicates the concept beautifully. What would otherwise seem absurd, that fallen, sinful man may become holy as God is holy, has been made possible through Jesus Christ, who is God incarnate. Naturally, the crucial Christian 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 even another god.
Through theoria, the knowledge of God in Jesus Christ, human beings come to know and experience what it means to be fully human (the created image of God); through their communion with Jesus Christ God shares Himself with the human race, in order to conform them to all that God is in knowledge, righteousness and holiness. Theosis also asserts the complete restoration of all people (and of the entire creation), in principle. This is built upon the understanding of the atonement put forward by Irenaeus, called "recapitulation".
For many fathers, theosis goes beyond simply restoring people to their state before the Fall of Adam and Eve, teaching that because Christ united the human and divine natures in his person, it is now possible for someone to experience closer fellowship with God than Adam and Eve initially experienced in the Garden of Eden, and that people 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.
All of humanity 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 human are not two persons but one; thus, a union is effected in Christ, between all of humanity and God. So, the holy God and sinful humanity 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 (podvig in Russian) to conform to the image of Christ. Without the struggle, the praxis, there is no real faith; faith leads to action, without which it is dead. One must unite will, thought and action to God's will, His thoughts and His actions. A person must fashion his life to be a mirror, a true likeness of God. More than that, since God and humanity are more than a similarity in Christ but rather a true union, Christians' lives are more than mere imitation and are rather a union with the life of God Himself: so that, the one who is working out salvation, is united with God working within the penitent both to will and to do that which pleases God. Gregory Palamas affirmed the possibility of humanity's union with God in His energies, while also affirming that because of God's transcendence and utter otherness, it is impossible for any person or other creature to know or to be united with God's essence. Yet through faith we can attain phronema, an understanding of the faith of the Church.
The journey towards theosis includes many forms of praxis. 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 (1 and 2). This unceasing prayer of the heart is a dominant theme in the writings of the Fathers, especially in those collected in the Philokalia.
Union with God, east and west
In western Catholic theology, theosis refers to a specific and rather advanced phase of contemplation of God.  The process of arriving to such a state, or moving toward it (as arrival there is not necessary for salvation), involves different types of prayer which are recognized as beneficial. Various stages of prayer life are recognized as being likely to occur should a person respond to faith by moving along the purgative, illuminative, and unitive ways. See ascetical theology.
Some western writers refer to theosis using the same implications given above (e.g., , ). It is common to find western writings that suggest that eastern spirituality manifests theosis and that by implication the west is lacking in this regard, but this is a case of rhetoric obscuring fact: under different terminology the western spiritual traditions, which also reach to the origins of Christianity (in the East), share the objective of sharing in the life of God. It is also necessary to recall that in the west there is a problematic form of ecumenism in vogue, in which people are quick to deny their own truths in order to appear to exalt the other. Some Catholic writers consider it lamentable that the term theosis is not used more extensively in western theology. 
It is, therefore, a mistake to attribute to Eastern Orthodoxy a special insight into the existence of the possibility of union with God: the theological difference between east and west is rhetorical. Whether or not eastern liturgies are more conducive to theosis is another matter; in the west there has been much discussion of the merits of the Novus Ordo Missae. The Tridentine Mass is available in hundreds of locations and is very much conducive, if for some faithful the Novus Ordo is not, to the kind of prayer life that leads one along toward theosis. Virtually all spiritual books of any consequence published in the west manifest overt awareness of all the issues comprised in theosis (some books may focus on specific stages and treat unitive themes more briefly).
Protestant use of the term "theosis"
Theosis as a concept is used among Methodists  especially in relation to the pietist movement and in the 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. In 1311 the Council of Vienne declared this notion, "that man in this present life can acquire so great and such a degree of perfection that he will be rendered inwardly sinless, and that he will not be able to advance farther in grace" (Denziger §471), to be a heresy. The Protestant conceptions of praxis, phronema, ascetical theology, and sacraments are quite different from Catholic and Orthodox understandings, but the use of the term theosis may illustrate a commonality of objective or hope.
Deification in Mormonism
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, most notably the doctrine of the Trinity.