The Sacrament of Baptism is often called "the door of the Church," because it is the first of the seven sacraments not only in time (since most Catholics receive it as infants) but in priority, since the reception of the other sacraments depends on it. It is the first of the three Sacraments of Initiation, the other two being the Sacrament of Confirmation and the Sacrament of Holy Communion. Once baptized, a person becomes a member of the Church.
Christ Himself ordered His disciples to preach the Gospel to all nations and to baptize those who accept the message of the Gospel. In His encounter with Nicodemus (John 3:1-21), Christ made it clear that baptism was necessary for salvation: "Amen, amen I say to thee, unless a man be born again of water and the Holy Spirit, he cannot enter into the kingdom of God." For Catholics, the sacrament is not a mere formality; it is the very mark of a Christian, because it brings us into new life in Christ.
While the Church has an extended rite of Baptism which is normally celebrated, which includes roles for both parents and godparents, the essentials of that rite are two: the pouring of water over the head of the person to be baptized (or the immersion of the person in water); and the words "I baptize you in the name of the Father, and of the Son, and of the Holy Spirit."
In the Catholic Church today, baptism is most commonly administered to infants. While some other Christians strenuously object to infant baptism, believing that baptism requires assent on the part of the person being baptized, the Eastern Orthodox, Anglicans, Lutherans, and other mainline Protestants also practice infant baptism, and there is evidence that it was practiced from the earliest days of the Church.
Since baptism removes both the guilt and the punishment due to Original Sin, delaying baptism until a child can understand the sacrament may put the child's salvation in danger, should he die unbaptized.
Adult converts to Catholicism from another non-Christian faith tradition also receive the sacrament. A person can only be baptized once as a Christian—if, say, he was baptized as a Lutheran, he cannot be re-baptized when he becomes a Catholic. The Christian will make a Catholic profession of faith to become a Catholic. It is improper to refer to someone coming from another Christian denomination as a convert. One converts from a non-belief of Jesus Christ as Messiah and Savior to a faithful follower.
While an adult can be baptized after proper instruction in the Faith, adult baptism normally occurs today as part of the Rite of Christian Initiation for Adults (RCIA) and is immediately followed by Confirmation and Communion.
Baptism has six primary effects, which are all supernatural graces:
The removal of the guilt of both Original Sin (the sin imparted to all mankind by the Fall of Adam and Eve in the Garden of Eden) and personal sin (the sins that we have committed ourselves).
The remission of all punishment that we owe because of sin, both temporal (in this world and in Purgatory) and eternal (the punishment that we would suffer in hell).
The infusion of grace in the form of sanctifying grace (the life of God within us); the seven gifts of the Holy Spirit (Wisdom, Understanding, Counsel, Fortitude, Knowledge, Piety, Fear of the Lord); and the three theological virtues (Faith, Hope, Charity).
Becoming a part of Christ.
Becoming a part of the Church - the Mystical Body of Christ on earth.
Enabling participation in the sacraments, the priesthood of all believers, and the growth in grace.