Jesus took with him Peter and James and his brother John and led them up a high mountain where they could be alone. There in their presence he was transfigured: his face shone like the sun and his clothes became as white as the light. Suddenly Moses and Elijah appeared to them; they were talking with him. Then Peter spoke to Jesus. ‘Lord,’ he said ‘it is wonderful for us to be here; if you wish, I will make three tents here, one for you, one for Moses and one for Elijah.’ He was still speaking when suddenly a bright cloud covered them with shadow, and from the cloud there came a voice which said, ‘This is my Son, the Beloved; he enjoys my favour. Listen to him.’ When they heard this the disciples fell on their faces overcome with fear. But Jesus came up and touched them. ‘Stand up,’ he said ‘do not be afraid.’ And when they raised their eyes they saw no one but only Jesus.
As they came down from the mountain Jesus gave them this order, ‘Tell no one about the vision until the Son of Man has risen from the dead.’
Source: Jerusalem Bible
Catechism of the Catholic Church
A foretaste of the Kingdom: the Transfiguration
554 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!”
555 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.”
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.
556 On the threshold of the public life: the baptism; on the threshold of the Passover: the Transfiguration. Jesus’ baptism proclaimed “the mystery of the first regeneration”, namely, our Baptism; the Transfiguration “is the sacrament of the second regeneration”: our own Resurrection.From now on we share in the Lord’s Resurrection through the Spirit who acts in the sacraments of the Body of Christ. The Transfiguration gives us a foretaste of Christ’s glorious coming, when he “will change our lowly body to be like his glorious body.” But it also recalls that “it is through many persecutions that we must enter the kingdom of God”:
Peter did not yet understand this when he wanted to remain with Christ on the mountain. It has been reserved for you, Peter, but for after death. For now, Jesus says: “Go down to toil on earth, to serve on earth, to be scorned and crucified on earth. Life goes down to be killed; Bread goes down to suffer hunger; the Way goes down to be exhausted on his journey; the Spring goes down to suffer thirst; and you refuse to suffer?”