Theosis, deification, is the Orthodox understanding of the goal of the Christian life. Salvation is not only the forgiveness of sins but transformation: the human person, by grace, coming to share in the very life of God. “He was made man,” the Fathers say, “that we might be made God”, not that we become God in essence, which is impossible, but that we are filled with His uncreated grace and light.
The distinction the Fathers draw is careful. God's essence remains forever beyond us, past every knowing; but His energies, His grace, His light, His very life reaching out to us, these we can truly share. This is the light the disciples saw on Mount Tabor at the Transfiguration: the uncreated glory of God, which the saints come to be filled with, so that they shine with a light not their own.
Theosis is not reserved for mystics; it is the ordinary vocation of every baptised person, worked out through prayer, the Mysteries, and the keeping of the commandments in love. It is why the Church exists and why she prays and fasts and feasts, that the human person, made in God's image, might grow at last into His likeness.