Yesterday was, in the Orthodox calendar, the feast of St Gregory Palamas (1296-1359), a monk of Mt Athos, an archbishop of Thessalonica, but above all a great theologian. Largely as the result of a prolonged dispute with Barlaam of Calabria, Gregory clarified and solidified one feature of the Eastern Church's teaching on the nature of God. God's transcendent aspect (his "essence") is unknowable by us because of the gulf inherent between Creator and creatures. However, we do encounter God in his "uncreated energies" (his immanent aspect), in which God reaches out to us in love and holds us and the entire universe in existence. The process of theosis, in which we become progressively more united with God, is a process of union with God's uncreated energies.

Illustrating these observations are two quotes, one from number 78 of Palamas' 150 Chapters, and the second from an article by Russian theologian Georges Florovsky.

Palamas

There are three realities in God, namely substance [essence], energy, and a Trinity of divine hypostases. Since it has been shown above that those deemed worthy of union with God so as to become one spirit with him...are not united to God in substance, and since all theologians bear witness in their statements to the fact that God is imparticipable in substance and the hypostatic union happens to be predicated of the Word and God-man alone, it follows that those deemed worthy of union with God are united to God in energy and that the spirit whereby he who clings to God is one with God is called and is indeed the uncreated energy of the Spirit and not the substance of God, even though Barlaam...may disagree.

Florovsky

Actually the whole teaching of St Gregory presupposes the action of the Personal God. God moves toward man and embraces him by his own "grace" and action without leaving that light unapproachable in which he eternally abides. The ultimate purpose of St Gregory's theological teaching was to defend the reality of Christian experience. Salvation is more than forgiveness. It is genuine renewal of man. And this renewal is not effected by the discharge, or release, of certain natural energies implied in man's own creaturely being, but by the "energies" of God himself, who thereby encounters and encompasses man, and admits him into communion with himself.