from catholicismabout.com
"The Necessity of Baptism:
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 Ghost, 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."
Note that Christ doesn't give any exceptions to the rule, though the RC church recognizes the baptism of desire and blood as well (neither of which babies can qualify for).
"Infant Baptism:
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."
"The Effects of the Sacrament:
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; and the three theological virtues.
- Becoming a part of Christ.
- Becoming a part of the Church, which is the Mystical Body of Christ on earth.
- Enabling participation in the sacraments, the priesthood of all believers, and the growth in grace"
If not for original sin, we would not baptise infants. We would wait until they were old enough to understand what it means to join the church. But, because we don't want them to die in a state of sin (original sin is a state, not an act), we baptise them.