Holy 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. 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: Baptism is the sacrament of regeneration through water in the word. (Catechism of the Catholic Church)
What does the Grace of Baptism accomplish?
Catholic Baptism does five things specifically.
It forgives all sins that may have been committed prior to a person’s baptism including original sin, mortal sins, and venial sins, and it relieves the punishment for those sins.
It makes the newly baptized person “a new creation.”
It turns the person into a newly adopted son of God and a member of Christ. Baptism incorporates one into the Church which is the body of Christ.
It brings someone into the flock of the faithful and brings them to share in the royal priesthood of Christ (1 Pet. 2:9-10). Catholic baptism gives a share in the common priesthood of all believers and it also brings about the sacramental bond of the unity of Christians. Paragraph 1271 of the Catechism says it best:
“Baptism constitutes the foundation of communion among all Christians, including those who are not yet in full communion with the Catholic Church: “For men who believe in Christ and have been properly baptized are put in some, though imperfect, communion with the Catholic Church. Justified by faith in Baptism, [they] are incorporated into Christ; they therefore have a right to be called Christians, and with good reason are accepted as brothers by the children of the Catholic Church. Baptism therefore constitutes the sacramental bond of unity existing among all who through it are reborn.”
Last, but certainly not in the least, baptism leaves and indelible spiritual mark (character) of belonging to Christ on the soul. Nothing you can do will take away this mark even if you sin a million times. Those sins may not grant you salvation, but you will always carry the mark of a Christian on your soul, therefore making re-baptism impossible.