In his dialogue with Nicodemus, Jesus taught that Baptism was necessary for salvation. "No one can enter the kingdom of God without being born of water and Spirit" (Jn 3:5). After his Resurrection, Jesus met with the eleven Apostles and gave them the commission to preach the Gospel and baptize, telling them, "Whoever believes and is baptized will be saved" (Mk 16:16).
The word baptism in its origins is Greek and means "immersion" and "bath." Immersion in water is a sign of death and emersion out of the water means new life. To bathe in water is also to undergo cleansing. Saint Paul sums up this truth when he says, "You were buried with him in baptism, in which you were also raised with him through faith in the power of God, who raised him from the dead" (Col 2:12).
The origin and foundation of Christian Baptism is Jesus. Before starting his public ministry, Jesus submitted himself to the baptism given by John the Baptist. The waters did not purify him; he cleansed the waters. "He comes to sanctify the Jordan for our sake . . . to begin a new creation through the Spirit and water" (St. Gregory Nazianzen, Liturgy of the Hours, I, 634).
Jesus' immersion in the water is a sign for all human beings of the need to die to themselves to do God's will. Jesus did not need to be baptized because he was totally faithful to the will of his Father and free from sin. However, he wanted to show his solidarity with human beings in order to reconcile them to the Father.
By commanding his disciples to baptize all nations, he established the means by which people would die to sin-Original and actual - and begin to live a new life with God.
-From the United States Catholic Catechism for Adults