Perhaps the best introduction in English to theosis--the Eastern Christian doctrine of how we attain union with God--is Norman Russell's Fellow Workers with God: Orthodox Thinking on Theosis (St Vladimir's Seminary Press, 2009). In straightforward, accessible prose, Russell outlines how the question has been addressed by the Fathers and contemporary Orthodox theologians; he also refers to the steadily increasing interest in deification by Roman Catholic, Anglican, and Protestant thinkers. Pages 169 and 170 contain a good summary of his argument. Theosis is not just for a monastic spiritual elite, and it furthermore underpins the liturgy.

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...as Metropolitan John Zizioulas has said, mystical experience is not to be identified solely with the extraordinary and unusual. The fact that theosis encompasses the whole of the economy of salvation means that it is intended for all believers without exception. To live theosis, then, means to lead our life in an eschatological perspective within the ecclesial community, striving through prayer, participation in the Eucharist, and the practice of the moral life to attain the divine likeness, being conformed spiritually and corporeally to the body of Christ until we are brought into Christ's identity and arrive ultimately at union with the Father. In simpler terms, it means for an Orthodox Christian to live as a faithful member of the Church, attending the Liturgy, receiving the sacraments and keeping the commandments. Nothing more--or less-than that.

The spirituality of the Orthodox Church is both liturgical and monastic. Liturgical spirituality takes full account of our corporeal nature. For the body is part of our identity. It is not something to be ignored or despised. The annual cycle of Great Feasts, particularly the Nativity, the Theophany [Epiphany], the Resurrection, the Ascension and the Transfiguration of Christ celebrate the transformation of humanity, body and soul, and its exaltation to heaven. On Holy Cross Day "the whole creation is set free from corruption". On Christmas Day "has God come upon earth, and man gone up to heaven". On the Forefeast of the Theophany Christ "opens the heavens, brings down the divine Spirit, and grants man a share of incorruption". On the Feast of the Transfiguration Christ "has changed the darkened nature of Adam, and filling it with brightness He has made it godlike". These feasts do not simply commemorate past events. With their eschatological dimension, frequently reinforced by the present tense of the verbs, they turn the worshiper "towards the future--towards the 'splendor of the Resurrection' at the Last Day, towards the 'beauty of the divine Kingdom' which all Christians hope eventually to enjoy."