“Holy Baptism is the basis of the whole Christian life, the gateway to life in the Spirit (vitae spiritualis ianua), 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.”

CCC 1213

Baptism in the Catholic Church is not just a symbolic action but confers grace to the one being baptized. Through Baptism, we are born into a new spiritual life in which we become the adopted children of God. Jesus intended it to be an integral and necessary part of being one of his followers.  As John’s gospel describes it, “Jesus answered [Nicodemus], ‘Truly, truly, I say to you, unless one is born of water and the Spirit, he cannot enter the kingdom of God.’ After this [conversation] Jesus and his disciples went into the land of Judea; there he remained with them and baptized” (John 3:5, 22).

Baptism is the sacrament that opens our soul to the flow of God’s infinite love for us.  It washes away our sins, grants us new life in Christ, and joins us to the Mystical Body of Christ.  With Confirmation and the Eucharist, Baptism is one of the sacraments of initiation.  These three lay the foundation of our Christian lives.  Any physical life involves birth, growth, and nourishment.  Our spiritual life involves the same:  birth (Baptism), growth (Confirmation), and nourishment (Eucharist).

According to John’s gospel, ““Jesus answered [Nicodemus], ‘Truly, truly, I say to you, unless one is born of water and the Spirit, he cannot enter the kingdom of God.’ After this [conversation] Jesus and his disciples went into the land of Judea; there he remained with them and baptized” (John 3:5, 22).

Just as do the other sacraments, Baptism makes visible an invisible reality, using physical signs to express our experience of God and his grace in our lives. These signs involve both a “thing” that is used to symbolize the sacramental flow of grace and words or gestures that signify the action of God.  In Baptism, the “thing” this is used is water, symbolizing cleansing and new life.  The words that explain this symbol to us are, “I baptize you in the Name of the Father, and of the Son, and of the Holy Spirit.”  These words reveal that God in the Trinity is both the source of life and our life’s goal.

In the eyes of the Catholic Church, any Baptism that uses water and the invocation of the Holy Trinity is a valid sacrament. So if a follower of a Christian church that performs Baptism to these standards wants to become Catholic, he doesn’t have to be re-baptized.