A foretaste of the kingdom: 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!"

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 thatyour Passion was voluntary, and proclaim to the world that you truly are the splendor of the Father.

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.” 

excerpts  from THE MYSTERIES OF CHRIST'S LIFE
in the Catechism of the Catholic Church

Read more:  Culminating Point of His Public Life