The short answer
Baptism is not merely a public symbol. Scripture describes it as God's sacramental action in which the believer is united to Christ's death and resurrection.
Jesus said
Jesus joins water and Spirit in the mystery of new birth and commands His apostles to baptize all nations.
John 3:5Matthew 28:19–20Mark 16:16
The apostles taught
The apostolic writings connect baptism with forgiveness, salvation, burial with Christ, and entrance into one Body.
Acts 2:38Romans 6:3–41 Peter 3:21
The Catholic synthesis
Baptism is the sacrament of faith and the ordinary gateway into Christian life. Its saving power comes entirely from Christ, not from human merit.