Are you unaware that we who were baptized into Christ Jesus were baptized into his death? We were indeed buried with him through baptism into death, so that, just as Christ was raised from the dead by the glory of the Father, we too might live in the newness of life. --Romans 6:3-4
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. St. 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...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...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. --United States Catholic Catechism for Adults, pps. 183 - 184.