Soteriology / Theosis
Soteriology is the doctrine of salvation. In the Orthodox Church, salvation is understood as theosis, the infinite process of becoming more and more like God. It is also termed deification or divinization, and its meaning is that the Christian may become more and more soaked with the divine life, becoming by grace what Christ is by nature. St. Athanasius the Great teaches that, "God became man so that man might become god." By participation in the incarnation, man becomes like Christ.
Salvation is a process which encompasses not only the whole earthly life of the Christian, but also the eternal life of the age to come. It is often described in terms of three stages-catharsis (purification), theoria (illumination) and theosis (divinization). Salvation is thus not only becoming sinless (purification), but it is also a progress in being filled with the divine light. Additionally, it is becoming so filled with God in union with Him that the Christian shines forth with the likeness of God, sometimes even literally becoming a bearer of the uncreated light.
It is only in and through Christ that man can be saved. Salvation cannot be earned, being a free gift from God. Its acquisition, however, requires man's cooperation with God, because God will not violate the free will of man. Thus, a life of repentance and participation in the sacraments is the means by which man cooperates with God. This cooperation is termed synergeia (synergy).