IN BRIEF
Christ's Transfiguration aims at strengthening the apostles' faith in anticipation of his Passion: the ascent on to 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).
Source: Catechism of the Catholic Church (Vatican.va)