As the foundation for the sacramental life, baptism:
Ushers us into the divine life
Cleanses us from sin
Initiates us as members of the Christian community.
Fundamentally, the ritual involves the priest pouring water on the candidate's head while saying: “I baptize you in the name of the Father, and of the Son and of the Holy Spirit" (CCC 1284).
The Sacrament of Baptism, like all Sacraments, includes symbols:
The waters of baptism recall Jesus’ own baptism by John the Baptist in the river Jordan. Water is a symbol of cleansing and renewal as we begin a new life in Christ. We are washed clean of original and actual sin.
At baptism we are anointed into the life of Christ as “priest, prophet and king.” A cross is traced on the candidate’s forehead as a reminder that we are inheritors of the Kingdom of God.
The baptismal candle is lit from the Paschal or Easter candle that stands in the church as a sign of Christ’s light in the world. At baptism, we receive the light of Christ and are called to share this light with others.
The white garment that is placed upon us at baptism is a symbol of Christ’s victory over death and his glorious resurrection. Likewise, the white garment or pall that is placed over the coffin at the time of death recalls our baptismal promises and reminds us that we are destined for eternal life.
Scripture & Baptism
Scripture repeatedly notes the importance of baptism as more than a mere symbol. Some of the relevant verses are:
"...[U]nless one is born of water and the Spirit, he cannot enter the kingdom of God." (John 3:5)
"But you were washed, you were sanctified, you were justified in the name of the Lord Jesus Christ and in the Spirit of our God." (1 Cor 6:11)
"Baptism... now saves you." (1 Peter 3:21)
"Amen, amen, I say to you, no one can enter the kingdom of God without being born of water and Spirit." (John 3:5)
"I confess one baptism . . ."
As we say in the Nicene Creed, “I confess one Baptism for the forgiveness of sins…” For this reason, a person can only be baptized once. Let's say, for example, that an adult who was baptized in another Christian denomination chooses to join the Catholic Church; s/he will not be "re-baptized".
Already dedicated to him through Baptism, the person who surrenders themselves to the God they love above all else thereby consecrates themselves more intimately to God's service and to the good of the church. --Catechism of the Catholic Church, no. 931