By Richard Rohr
The Orthodox teaching of divinization, or theosis, according to Pope John Paul II, is perhaps the greatest gift of the Eastern Church to the West, but one that has largely been ignored or even denied.  The Eastern fathers of the Church believed that we could experience real and transformative union with God. This is in fact the supreme goal of human life and the very meaning of salvation—not only later, but now, too. Theosis refers to the shared deification or divinization of creation, particularly with the human soul where it can happen consciously and lovingly.
St. Gregory of Nazianzus (330–390) emphasized that deification does not mean we become God, but that we do objectively participate in God’s nature. We are created to share in the life-flow of Trinity. Salvation isn’t about replacing our human nature with a fully divine nature but growing within our very earthiness and embodiedness to live more and more in the ways of love and grace, so that it comes “naturally” to us and is our deepest nature.
This does not mean we are humanly or perfectly whole or psychologically unwounded, but it has to do with an objective identity in God that we can always call upon and return to without fail. A doctrine of divinization is the basis for hope and growth. Divinized people live in a grateful state of awareness, recognizing their undeserved union with God, but that does not always mean their stage of human development is without very real limits and faults. This is a distinction that the West, with its dualistic mind, seemed unable to make.
This is how a few ancients and contemporaries understand theosis:
Athanasius of Alexandria (296–373): “The Son of God became man that we might become god. . . . [It is] becoming by grace what God is by nature.”  Athanasius is almost directly quoting St. Irenaeus (125–203) who taught the same.
Maximus the Confessor (580–662): “The saints become that which can never belong to the power of nature alone, since nature possesses no faculty capable of perceiving what surpasses it.” 
Olivier Clément (1921–2009), a favorite Orthodox scholar, writes: “The purpose of the incarnation is to establish full communion between God and humanity so that in Christ humanity may find adoption and immortality, often called ‘deification’ by the Fathers: not by emptying out our human nature but by fulfilling it in the divine life, since only in God is human nature truly itself.” 
- A. McGuckin (born 1952), another Orthodox scholar: “In speaking of fullness of communion as the ‘true life’ of the creature, deification language shows that the restoration of communion is at root one and the same movement and motive of the God who seeks to disburse the gift of the fullness of life to [God’s] rational creatures.” 
Full salvation is finally universal belonging and universal connecting. Another word for that is quite simply “heaven.”
 Pope John Paul II, Orientale Lumen (The Light of the East), Apostolic Letter on the Eastern Churches (May 2, 1995). Text available at https://w2.vatican.va/content/john-paul-ii/en/apost_letters/1995/documents/hf_jp-ii_apl_19950502_orientale-lumen.html.
 Athanasius of Alexandria, De Incarnatione (On the Incarnation), 54. My paraphrase.
 Maximus the Confessor, Questions to Thalassius, 22. See John Meyendorff, St. Gregory Palamas and Orthodox Spirituality (St. Vladimir’s Seminary Press: 1974), 39.
 Olivier Clément, The Roots of Christian Mysticism, 2nd ed. (New City Press: 2013), 37.
 J. A. McGuckin, “The Strategic Adaptation of Deification in the Cappadocians,” Partakers of the Divine Nature: The History and Development of Deification in the Christian Tradition, eds. Michael J. Christensen and Jeffery A. Wittung (Baker Academic: 2007), 96. Partakers of the Divine Nature is the best overview I have read on the subject of theosis.