Theosis is the name of a doctrine that originated in the Patristic period. It concerns the claim that Christians are able to be transformed through the power of the Holy Spirit to become more morally and willfully intact; that is, believers are given a co operant grace to participate in the restoration of the moral image of God. It is the “call to man to become holy and seek union with God, beginning in this life and later consummated in the resurrection” and, though often equated with Eastern Orthodox or Catholic theology in contemporary thought, is frequently conceived as something akin to sanctification in Protestant theology.
The word theosis literally means “ingodded”, though it is often translated as deification or divinization. The doctrine is understood to be grounded in Scriptures (Psalms 82:6, John -35, 2 Peter 1:4, 1 John 3:1-2) and in the Apostolic Tradition according to its principal proponents (Origen, Clement, Ephrem, Macarius, Gregory of Nyssa, Maximus the Confessor). Over time, the Eastern Orthodox doctrine of theosis became defined as a "union (of energies) without confusion (of essence)" in which the essential distinction between Creator and creature eternally remains. As Orthodox Bishop Kalistos Ware writes: "In the Age to come, God is 'all in all,' but Peter is Peter and Paul is Paul."
The doctrine began to gain credence with the axiom-driven support of such enthusiasts as Ireneus, who claimed that “if the Word was made man it is that men might become gods”, and Athenasius, who marked the purpose of the Incarnation that “God became man so that man might become God.” Ireneus, in particular, saw “salvation as transformation of humans into partakers of the divine nature” and in later days his thoughts were often accompanied by scriptures such as 1 Peter 1.4 (“Through these he has given us his very great and precious promises, so that through them you may participate in the divine nature and escape the corruption in the world caused by evil desires”).
The transformation brought about through theosis is multi-faceted, and theosis itself furthers our image and understanding of God. Through theoria, the knowledge of God in Christ Jesus, believers will “come to know and experience what it means to be fully [in the created image of God].” This is a restoration of the moral image of God in humanity. Through communion with Christ, “God shares Himself with humanity in order to conform them to all that God is in knowledge, righteousness, and holiness,” which is the process by which believers are made holy in their character. St. Maximus the Confessor, a proponent and defender of the doctrine of theosis c. 580-682, wrote:
"A sure warrant for looking forward with hope to deification of human nature is provided by the incarnation of God, which makes man god to the same degree as God Himself became man. For it is clear that He who became man without sin (cf. Heb. ) will divinize human nature without changing it into the divine nature, and will raise it up for His own sake to the same degree as He lowered Himself for man's sake. This is what
The contemporary view of western Catholics differs slightly from this perspective, however, and sees theosis as a “specific and advanced form of contemplative prayer,” while that of modern Protestants, with the exception of Methodists and some pietists, tends to ignore the term theosis in favor of some version of the doctrine of sanctification or, more commonly, entire sanctification.
Yet those who hold to the doctrine of theosis would mark a distinction between their understanding of divinization as substantively more than the Protestant doctrine of sanctification. For, while theosis boasts no ontological change – i.e. humanity actually becoming fully divine in nature – there is an ontological participation by which there is a “very real participation in the divine life.” “The Orthodox hope of salvation,” says scholar Ross Aden, “in its broadest sense is more than hope of a divine sentence of 'not guilty' or even of a beatific vision; it is `human participation in the being of God . . . a total sharing in the Triune life.” According to some scholars the western churches come closest to this kind of understanding through the use of the phrase “the imitation of Christ”, the “imitation”, however, is of a different quality than the “manifestation of the energies of the Holy Spirit.”
Theosis is, perhaps, one way in which Protestants [and particularly Evangelicals] may rediscover a healthy emphasis on the ongoing progress of the Christian life which seems all-too-absent from our popular teachings. Though to some it may seem like simply a “re-branding” of Wesleyan sanctification doctrine, I believe there is a richness to this theology that may not be fully represented in our understanding of sanctification; and, one in which we find great meaning and encouragement to follow Christ.
 Bishop Kallistos Ware, The
 Bishop Kallistos Ware, The
 Against Heresies, Bk. V. Pref. col. 1035
 On the Incarnation of the Word, Bk. IV. par 65
 Roger E.Olson, The Story of Christian Theology: Twenty Centuries of Tradition and Reform. (Downer’s Grove: InterVarsity, 1999), 77.
 PHILOKALIA Volume II, page 178
 Ross Aden, "Justification and Sanctification: A Conversation Between Lutheranism and Orthodoxy," St. Vladimir's Theological Quarterly 38:1 (1994):96-98.
 Cf. http://www.bethel.edu/~rakrob/files/THEOSIS2.html#23 Accessed
 http://www.bethel.edu/~rakrob/files/THEOSIS2.html#23. Accessed