St Gregory Palamas (1296-1359), Archbishop of Thessaloniki, was the defender of the Hesychasts. He upheld the doctrine that the human body played an important part in prayer, and he argued that the Hesychasts did indeed experience the Divine and Uncreated Light of Tabor. To explain how this was possible, St. Gregory developed the distinction between the essence and the energies of God. He set Hesychasm on a firm dogmatic basis, by integrating it into Orthodox theology, and by showing how the Hesychast vision of Divine Light in no way undermined the doctrine that God can not be comprehended. His teachings were confirmed by the local councils held in Constantinople in 1341 and 1351.
St. Gregory began by reaffirming the Biblical doctrine of man and of the Incarnation; i.e. the whole man, united in body and soul, was created in the image of God, and Christ, by taking a human body at the Incarnation, has 'made the flesh an inexhaustible source of sanctification'. The Hesychasts, so he argued, in placing emphasis on the body's part in prayer, are not guilty of a gross materialism but are simply remaining faithful to the Biblical doctrine of man as a unity. Christ took human flesh and saved the whole man; therefore it is the whole man that prays to God.
How is it possible for man to know God and, at the same time, affirm that God is by nature unknowable? St. Gregory answered this question by quoting St. Basil the Great who said "We know our God from His energies, but we do not claim that we can draw near to His essence. For His energies come down to us, but His essence remains unapproachable". St. Gregory added "God is not a nature, for He is above all beings.... No single thing of all that is created has or ever will have even the slightest communion with the supreme nature, or nearness to it". Even though God's essence may be remote from us, He has revealed Himself through His energies (or grace). These energies do not exist apart from God, but are God Himself in His action and revelation to the world. It is through these energies that God enters into a direct and immediate relationship with us. When we say that the saints are 'deified' by the grace of God, we mean that they have a direct experience of God Himself through his energies (or grace), not in His essence.
The vision of Light that Hesychasts receive is the same Light that surrounded Christ on Mount Tabor. It is a true vision of God in His divine energies.