Catechism of the Catholic Church §568 — excerpt
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.
This translation is under copyright, so only the cited words are quoted here. Follow the link for the full text. Translation: English translation of the Catechism of the Catholic Church, 2nd ed. (Libreria Editrice Vaticana), as posted at vatican.va.