Jesus had gone with his disciples Peter, James, and John to Mount Tabor. Christ's appearance was changed while they watched into a glorious radiant figure. There appeared Elijah and Moses, speaking with Jesus. The disciples were amazed and terribly afraid.
This event shows forth the divinity of Christ, so that the disciples would understand after his Ascension that He was truly the radiant splendor of the Father, and that his Passion was voluntary (Gospel of Mark 9:2-9). It also shows the possibility of our own theosis.
This event was the subject of some debates between Gregory Palamas and Barlaam of Calabria. Barlaam believed that the light shining from Jesus was created light, while Gregory maintained the disciples were given grace to perceive the uncreated light of God. This supported Gregory's larger argument that although we cannot know God in His essence, we can know Him in his energies, as He reveals Himself.