Almighty, eternal God, when the Spirit descended upon Jesus at His baptism in the Jordan, You revealed Him as Your own beloved Son. Keep us, Your children born of water and the Spirit, faithful to our calling.
Christ's Baptism - from the Catechism of the Catholic Church
All the Old Covenant prefigurations find their fulfillment in Christ Jesus. He begins his public life after having himself baptized by Saint John the Baptist in the Jordan. After His resurrection Christ gives this mission to His apostles: "Go therefore and make disciples of all nations, baptizing them in the name of the Father and of the Son and of the Holy Spirit, teaching them to observe all that I have commanded you."
Our Lord voluntarily submitted Himself to the baptism of Saint 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."
In His Passover Christ opened to all men the fountain of Baptism. He had already spoken of His Passion, which He was about to suffer in Jerusalem, as a "Baptism" with which He had to be baptized. The blood and water that flowed from the pierced side of the crucified Jesus are types of Baptism and the Eucharist, the sacraments of new life. From then on, it is possible "to be born of water and the Spirit" in order to enter the Kingdom of God.
See where you are baptized, see where Baptism comes from, if not from the cross of Christ, from His death. There is the whole mystery: He died for you. In Him you are redeemed, in Him you are saved. (1223-1225)