Jesus’ public life begins with his baptism by John in the Jordan. John preaches “a baptism of repentance for the forgiveness of sins”. A crowd of sinners – tax collectors and soldiers, Pharisees and Sadducees, and prostitutes – come to be baptized by him. “Then Jesus appears.” the Baptist hesitates, but Jesus insists and receives baptism. Then the Holy Spirit, in the form of a dove, comes upon Jesus and a voice from heaven proclaims, “This is my beloved Son.” This is the manifestation (“Epiphany”) of Jesus as Messiah of Israel and Son of God.
The baptism of Jesus is on his part the acceptance and inauguration of his mission as God’s suffering Servant. He allows himself to be numbered among sinners; he is already “the Lamb of God, who takes away the sin of the world”. Already he is anticipating the “baptism” of his bloody death. Already he is coming to “fulfil all righteousness”, that is, he is submitting himself entirely to his Father’s will: out of love he consents to this baptism of death for the remission of our sins.234 The Father’s voice responds to the Son’s acceptance, proclaiming his entire delight in his Son. The Spirit whom Jesus possessed in fullness from his conception comes to “rest on him”. Jesus will be the source of the Spirit for all mankind. At his baptism “the heavens were opened” – the heavens that Adam’s sin had closed – and the waters were sanctified by the descent of Jesus and the Spirit, a prelude to the new creation.
Through Baptism the Christian is sacramentally assimilated to Jesus, who in his own baptism anticipates his death and resurrection. the Christian must enter into this mystery of humble self-abasement and repentance, go down into the water with Jesus in order to rise with him, be reborn of water and the Spirit so as to become the Father’s beloved son in the Son and “walk in newness of life”:
Let us be buried with Christ by Baptism to rise with him; let us go down with him to be raised with him; and let us rise with him to be glorified with him. Everything that happened to Christ lets us know that, after the bath of water, the Holy Spirit swoops down upon us from high heaven and that, adopted by the Father’s voice, we become sons of God.
From the Catechism of the Catholic Church 535-537