But they did not understand this saying;
its meaning was hidden from them
so that they should not understand it,
and they were afraid to ask him about this saying.
-- Lk 9:43b-45
From the day Peter confessed that Jesus is the Christ, the Son of the living God, the Master "began to show his disciples that he must go to Jerusalem and suffer many things. . . and be killed, and on the third day be raised." Peter scorns this prediction, nor do the others understand it any better than he. In this context the mysterious episode of Jesus' Transfiguration takes place on a high mountain, before three witnesses chosen by himself: Peter, James and John. Jesus' face and clothes become dazzling with light, and Moses and Elijah appear, speaking "of his departure, which he was to accomplish at Jerusalem". A cloud covers him and a voice from heaven says: "This is my Son, my Chosen; listen to him!"
-- CCC 554