Catechism of the Catholic Church §556 — excerpt
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.
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.
Also cites: St. Thomas Aquinas, STh III, 45, 4, ad 2 · St. Augustine, Sermo 78, 6: PL 38, 492-493