Catechism of the Catholic Church
a and b. Para. 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" (Lk 24:26). Moses and Elijah had seen God's glory on the Mountain; the Law and the Prophets had announced the Messiah's sufferings (Cf. Lk 24:27). Christ's Passion is the will of the Father: the Son acts as God's servant (Cf. Isa 42:1); the cloud indicates the presence of the Holy Spirit. . . .
Para. 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 (St. Thomas Aquinas, STh III, 45, 4, ad 2). 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 . . ."
Para. 568: Christ's Transfiguration aims at strengthening the apostles' faith in anticipation of his Passion: the ascent onto the "high mountain" prepares for the ascent to Calvary. . . .