Catechism of the Catholic Church §1286 — excerpt
In the Old Testament the prophets announced that the Spirit of the Lord would rest on the hoped-for Messiah for his saving mission. The descent of the Holy Spirit on Jesus at his baptism by John was the sign that this was he who was to come, the Messiah, the Son of God.
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.