The monk, archbishop, and theologian Gregory Palamas (1296–1359) stood up to defend the authenticity of the hesychasts' claims against Barlaam's attacks. Between 1338 and 1341, Palamas wrote For the Defense of Those Who Practice Sacred Quietude defending the legitimacy of the hesychast experience. Given that this defense was shared across three books with three treatises in each, the work came to be known as The Triads. And it's in The Triads where Palamas shares his famous distinction between God's essence and God's energies.
Palamas centers his defense on the experience of Peter, James, and John on Mount Tabor when they beheld Jesus' transfiguration. On the mountian the apostles behold, directly, the glory of God. This glory, Palamas continues, is the same light the hesychasts were experiencing. "Do you not see," Palamas writes, "how this light shines even now in the hearts of the faithful and perfect?"
Against this view, critics like Barlaam were setting themselves up as "enemies of such an illumination" by arguing that "all the lights which God has manifested to the saints are only symbolic apparitions, allusions to immaterial and intelligible realities."
Again, Barlaam had some Biblical support for his contentions. There was also the strong witness of the apophatic tradition and its influence upon scholastic theology. As a scholastic theologian, and like Thomas Aquinas, Barlaam argued that God's essence was unknowable. Consequently, it was impossible for the hesychasts to gaze directly upon God's essence.
Palamas was quick to agree with Barlaam on this point. No one can see or behold God's essence. In good apophatic fashion, Palamas agrees that God's essence "transcends all affirmation and negation." God does not "allow Himself to be seen in His superessential essence."
But if that's true, what were the hesychasts seeing?
Palamas argues that what the hesychasts beheld, the same light seen by the apostles on Mount Tabor, was not God’s essence but God’s uncreated energy. By “energy” Palamas means God’s direct and uncreated activities, His operations by which He is present and active in the world. The names of these energies are various: virtue, goodness, power, grace, wisdom, illumination, providence, deification. Even existence itself. Palamas uses the metaphor of the sun to explain the contrast. There is a difference between what the sun is in itself (its essence) and the light/energy it radiates which we can enjoy and share.Moreover, the Holy Fathers affirm unanimously that it is impossible to find a name to manifest the nature of the uncreated Trinity, but the names belong to the energies...He Who is beyond every name is not identical with what He is named; for the essence and energy of God are not identical.
But just as one cannot see fire, if there is no matter to receive it, nor any sense organ capable of perceiving its luminous energy, in the same way one cannot contemplate deification if there is no matter to receive the divine manifestation. But if with every veil removed it lays hold of appropriate matter, that is of any purified rational nature, freed from the veil of manifold evil, then it becomes itself visible as a spiritual light. "The prize of virtue," it is said, "is to become God, to be illumined by the purest of lights, by becoming a son of that day which no darkness can dim. For it is another Sun which produces this day, a Sun which shines forth the true light. And once it has illumined us, it no longer hides in the West, but envelops all things with its powerful light. It grants an eternal and endless light to those worthy, and transforms those who participate in this light into other suns."

