Baptism is necessary for the salvation of all men, because Christ has said: “Unless a man be born again of water and the Spirit, he cannot enter into the kingdom of God.”
From the time of Christ this has been the unequivocal teaching of the Church. The reason lies in the fact that only Baptism can remit original sin; no one with any taint can enter heaven.
Those who through no fault of their own have not received the sacrament of Baptism can be saved through what is called baptism of blood or baptism of desire.
Either baptism of desire or of blood entitles one to the possesion of heaven, just as baptism by water. However, only baptism by water imprints a character on the soul, and so entitles one to the reception of the other sacraments.
1. An unbaptized person receives the baptism of blood when he suffers martyrdom for the faith of Christ.
One who lays down his life for Christ or some Christian virtue is said to have received baptism of blood. Our Lord promised, “He who loses his life for my sake will find it” (Matthew 10:39). Thus the Holy Innocents whom Herod slew out of hatred for the Infant Jesus received the baptism of blood.
2. An unbaptized person receives “baptism of desire” when he loves God above all things, is sorry for his sins, and ardently longs for Baptism when it is impossible to receive it; or when not knowing the necessity of Baptism, sincerely wishes to do all required for salvation.