Catechism of the Catholic Church

For a moment Jesus discloses his divine glory, confirming Peter's confession. He also reveals that he will have to go by the way of the cross at Jerusalem in order to "enter into his glory." Moses and Elijah had seen God's glory on the Mountain; the Law and the Prophets had announced the Messiah's sufferings. Christ's Passion is the will of the Father: the Son acts as God's servant; the cloud indicates the presence of the Holy Spirit. "The whole Trinity appeared: the Father in the voice; the Son in the man; the Spirit in the shining cloud." (2576, 2583, 257) > You were transfigured on the mountain, and your disciples, as much as they were capable of it, beheld your glory, O Christ our God, so that when they should see you crucified they would understand that your Passion was voluntary, and proclaim to the world that you truly are the splendor of the Father.