Baptism is the removal of original sin by an infusion of the Holy Spirit, Who enters the soul of the baptized and imparts to it sanctifying grace. A baptized person is justified and made righteous by partaking in the justice of God, which removes all guilt of sin and its punishment.
Baptism is accomplished in three ways: by water, by blood (or martyrdom) and by desire. Only baptism of water, called the sacrament of baptism, impresses a permanent baptismal character on the soul, enabling a person to receive the other sacraments. The Catechism of the Catholic Church (CCC) in paragraph 1258 speaks of all three means of baptism: "This Baptism of blood, like the desire for Baptism, brings about the fruits of Baptism without being a sacrament." Baptism of blood occurs when a person willingly dies for Christ or some Christian virtue. Martyrdom of a formerly unbaptized person remits all of his sins and ushers his soul into Heaven.
In his 13th-century Summa Theologica, St. Thomas Aquinas calls the desire for baptism a baptism of repentance: "In like manner a man receives the effect of Baptism by the power of the Holy Ghost, not only without Baptism of Water, but also without Baptism of Blood: forasmuch as his heart is moved by the Holy Ghost to believe in and love God and to repent of his sins: wherefore this is also called Baptism of Repentance."
In John 3:5 Christ declares, "Unless a man be born again of water and the Spirit, he cannot enter into the kingdom of God." Saint Thomas explains, "The other two Baptisms are included in the Baptism of Water, which derives its efficacy, both from Christ's Passion and from the Holy Ghost. Consequently for this reason the unity of Baptism is not destroyed."
In a letter approved by Pope Pius XII in 1949, the Holy Office states that a desire for baptism may be "implicit" but to "produce its effect" must be rooted in "supernatural faith" and "animated by perfect charity."
Find out why being called Catholic isn't enough for salvation in episode 11 of Church Militant's Premium show Baptize All Nations—Evangelization and Eternal Salvation.