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, teaching them to observe all that I have commanded you.
Matthew 28:19-20

1213 Holy Baptism is the basis of the whole Christian life, the gateway to life in the Spirit (vitae spiritualis ianua), and the door which gives access to the other sacraments. Through Baptism we are freed from sin and reborn as sons of God; we become members of Christ, are incorporated into the Church and made sharers in her mission.

1227 According to the Apostle Paul, the believer enters through Baptism into communion with Christ's death, is buried with him, and rises with him: 

Do you not know that all of us who have been baptized into Christ Jesus were baptized into his death? We were buried therefore with him by baptism into death, so that as Christ was raised from the dead by the glory of the Father, we too might walk in newness of life.

The baptized have "put on Christ."  Through the Holy Spirit, Baptism is a bath that purifies, justifies, and sanctifies.

1250 Born with a fallen human nature and tainted by original sin, children also have need of the new birth in Baptism to be freed from the power of darkness and brought into the realm of the freedom of the children of God, to which all men are called ... The Church and the parents would deny a child the priceless grace of becoming a child of God were they not to confer Baptism shortly after birth.

1257 The Lord himself affirms that Baptism is necessary for salvation... Baptism is necessary for salvation for those to whom the Gospel has been proclaimed and who have had the possibility of asking for this sacrament.  The Church does not know of any means other than Baptism that assures entry into eternal beatitude.