In today's Gospel we hear John the Baptist contrast his baptism of repentance with the baptism that Jesus will inaugurate. John the Baptist says that he has baptised with water, but that the one who is to come will baptise with the Holy Spirit. John's baptism was not yet a Christian baptism; it was a preparation for the Christian Baptism we celebrate today, and through which sins are forgiven and the gift of the Holy Spirit is received. In accepting John's baptism, Jesus, though sinless, united himself with all sinners.
The baptism of Jesus is reported in each of the three Synoptic Gospels—Matthew, Mark, and Luke. Clearly, Jesus' baptism was an event of great significance for Jesus and for the early Christian community. Mark and Luke report the story from Jesus' perspective; the voice from heaven is addressed to Jesus. In Matthew's Gospel, the voice from heaven speaks to all who are present. The descent of the Holy Spirit on Jesus at his baptism shows that something new is beginning through the baptism and ministry of Jesus.
The baptism of Jesus is considered an important manifestation of God in the person of Jesus, another epiphany. Jesus' baptism inaugurates his mission. Mark's Gospel moves quickly from the report of Jesus' baptism to Jesus' temptations in the desert to his ministry in Galilee after John's arrest. The end of the ministry of John the Baptist is the beginning of Jesus' ministry. In an analogous way, our Baptism inaugurates our mission as Christians.
Surprisingly, on this the last day of the Christmas season the Gospel does not tell a story from Jesus' childhood. Instead the Gospel reveals Jesus' relationship to God: the Son of Mary and Joseph is also God's own Son. We believe that through Baptism we are also made children of God.