"Nothing in theosis (being formed into God likeness) is the product of human nature, for nature cannot comprehend God. It is only the mercy of God that has the capacity to endow theosis unto the existing... In theosis man (the image of God) becomes likened to God, he rejoices in all the plenitude that does not belong to him by nature, because the grace of the Spirit triumphs within him, and because God acts in him" (Letter 22 to Thalassius, St Maximus the Confessor, ca 611-641).
St Maximus is the champion of the doctrine of "essence and energy," a teaching that has been almost completely ignored in the Western Church. In this teaching he points out the distinction and similitude that exists between the essence of a being, and the energy of the being. The difference between the two is primarily that "essence" is the proper person or being, and the "energy" is what the being produces or creates. As an example he sates that a person is a being, and the voice that emanates from that person is their energy. When another hears the persons voice, the hearer is affected by the person speaking and in doing so they partake in the speaker, yet at the same time they do not take on the speaker's essence. Another would be this: if a sharp sword were heated by a flame, and then made to cut an object, the heat of the flame transferred onto the sword becomes the property of the sword which would burn the object, yet the sword does not become the flame. Therefore, if we ask what what burned the object, the sword or the flame, the answer would be the flame through the sword. This understanding of essence and energy is the basis for the eastern view of sanctification. When the energy of God acts upon the Christian and it affects the Christian's essence to the point that the energy of God becomes part of person's essence; body, mind, and soul. This is quite different from the western teaching that what is affected is merely the mind and the heart. In theosis, the whole person is transformed by God's energy into the likeness of God.
Now may the God of peace himself sanctify you completely, and may your whole spirit and soul and body be kept blameless at the coming of our Lord Jesus Christ. He who calls you is faithful; he will surely do it. 1 Thess 5: 23-24