Catechism of the Catholic Church
e. 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. Christ, Head of the Church, manifests what his Body contains and radiates in the sacraments: "the hope of glory" (Col 1:27; cf. St. Leo the Great, Sermo, 51, 3: PL 54, 310c).
Para. 555: 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).
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.