Jesus took Peter, James, and his brother, John, and led them up a high mountain by themselves. And he was transfigured before them; his face shone like the sun and his clothes became white as light. And behold, Moses and Elijah appeared to them, conversing with him. (Matthew 17:1-3)
"Christ is transfigured, not by changing into something he never was before, but by revealing to his disciples what he truly was, in opening their eyes."
--Saint John Damascene