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 (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.

Accounts of the Transfiguration are found in the Bible: Matthew 17:1-8, Mark 9:2-9, Luke 9:28-36, and II Peter 1:16-19.

source: OrthodoxWiki

Matthew 17   (New King James Version)

 ¹ Now after six days Jesus took Peter, James, and John his brother, led them up on a high mountain by themselves; ² and He was transfigured before them. His face shone like the sun, and His clothes became as white as the light. ³ And behold, Moses and Elijah appeared to them, talking with Him. ⁴ Then Peter answered and said to Jesus, “Lord, it is good for us to be here; if You wish, let us^[a] make here three tabernacles: one for You, one for Moses, and one for Elijah.”
⁵ While he was still speaking, behold, a bright cloud overshadowed them; and suddenly a voice came out of the cloud, saying, “This is My beloved Son, in whom I am well pleased. Hear Him!” ⁶ And when the disciples heard it, they fell on their faces and were greatly afraid. ⁷ But Jesus came and touched them and said, “Arise, and do not be afraid.” ⁸ When they had lifted up their eyes, they saw no one but Jesus only.