Baptism - The 'Sacrament of Baptism' is the basis of the whole Christian life, the gateway to life in the Spirit, and the door which gives access to the other sacraments. Baptism brings one into faith. "I baptize you in the name of the Father, and of the Son and of the Holy Spirit." Through Baptism we are freed from sin and reborn as children of God; we become members of Christ, and are incorporated into the Church and made sharers in the Church's mission; "Baptism is the sacrament of regeneration through water in the word."

Baptism, comes from the Greek word 'baptizein' meaning to "plunge" or "immerse." to "plunge" into the water symbolizes the individual's burial into Christ's death, from which we also rise up by resurrection with Him, as a "new creation."

"Baptism is God's most beautiful and magnificent gift... We call it gift, grace, anointing, enlightenment, garment of immortality, bath of rebirth, seal, and the most precious gift. It is called gift because it is conferred on those who bring nothing of their own; grace since it is given even to the guilty; baptism because sin is buried in the water; anointing for it is priestly and royal as are those who are anointed; enlightenment because it radiates light; clothing since it veils our shame; bath because it washes; and seal as it is our guard and the sign of God's Lordship." St. Gregory of Nazianzus.

Christ sent his apostles so that "repentance and forgiveness of sins should be preached in his name to all nations." "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."

Baptism:
--cleanses one from sin
--gives birth to life in the spirit
--incorporates the individual into the Church community 

Cleansing from sin:
The principal gesture of baptism is that of bathing. Originally, the one baptized was plunged completely into the water, as Jesus was in the Jordan River; but today most people are baptized by having water poured over their heads. This external bath signifies and brings about an interior cleansing from sin.

All sin is removed by this sacrament: original and actual, mortal and venial -- and not only the guilt of sin but also whatever punishment is due for it. The recipient may have lived a life of utter wickedness; but if he/she has sincerely repented and adhered to Christ by faith, all of this is washed away in the waters of baptism. Were he/she to die immediately, he/she would be admitted directly into heaven like the "good thief" to whom Jesus said, "Today you will be with me in paradise" (Luke 23:43). For in the sacraments, it is not the merits of the recipient but the grace of Christ, that produces the effect; and this grace is infinitely more abundant than all the sins in the world (Romans 5:15-17).

Life in the Spirit
The cleansing from sin in baptism is brought about by the Holy Spirit who is poured into the recipient, making him/her a temple of the Holy Spirit (1 Cor 6:19). More than a temple: they become a child of God, sanctified by grace and alive with the life of the Blessed Trinity. Hence baptism can be called a rebirth, as in Jesus' words to Nicodemus, "Unless a man is born again of water and the Spirit, he cannot enter the kingdom of God (John 3:5).

Incorporation into the Church
By this sacrament, a person becomes a member of the Body of Christ. Jesus saves us by gathering us into a people; that is why baptism is not just a personal act of repentance of sin and faith in Christ; it is the act of admittance into the People of God.

http://www.usccb.org/catechism/text/pt2sect2.shtml

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