Christ's Baptism
Our Lord voluntarily submitted himself to the baptism of St. John, intended for sinners, in order to "fulfill all righteousness." Jesus' gesture is a manifestation of his self-emptying. The Spirit who had hovered over the waters of the first creation descended then on the Christ as a prelude of the new creation, and the Father revealed Jesus as his "beloved Son."
Source: Catechism of the Catholic Church (Vatican.va)