The Sacrament of Baptism is not just a symbol of an inward disposition; it is an outward act that signifies what is actually taking place spiritually. The water used to physically cleanse the baptized is a sign necessary for the cleansing of the soul to take place. Many Christian churches believe that baptism is more or less a social act that displays the inner disposition of the baptized, which is why most non-Catholic Christians are not baptized as infants. But the Catholic Church holds that as a Sacrament, Baptism is necessary for the spiritual changes to take place.
There are also clear indications of Infant Baptism in the Bible: In the New Testament we read that Lydia was converted by Paul’s preaching and that “She was baptized, with her household” (Acts 16:15). The Philippian jailer whom Paul and Silas had converted to the faith was baptized that night along with his household. We are told that “the same hour of the night . . . he was baptized, with all his family” (Acts 16:33). And in his greetings to the Corinthians, Paul recalled that, “I did baptize also the household of Stephanas” (1 Cor. 1:16). In all these cases, whole households or families were baptized. This means more than just the spouse; the children too were included. Certainly there were children younger than the age of reason in some of the households that were baptized. Furthermore, given the New Testament pattern of household baptism, if there were to be exceptions to this rule (such as infants), they would be explicit.
These things together with main effects of baptism mentioned above is why, in the eyes of the Catholic Church, Infant Baptism is crucial. Infants are born with original sin which separates them from the Body of Christ. Baptism will unite them as adopted children of God as well as give them the graces necessary to grow in their faith as they grow older.
Baptism leaves an indelible mark on one’s soul, forever marking them members of Christ’s Body. The Catholic Church recognizes Baptisms from all Christian churches as valid so long as they use water and the invocation of the Holy Trinity: “I baptize you in the name of the Father and of the Son and of the Holy Spirit.” If a follower of a Christian church was baptized to these standards wants to become Catholic, he or she does not need to be re-baptized.