New City Catechism questions 44 and 45:
What is baptism?
Baptism is the washing with water in the name of the Father, the Son, and the Holy Spirit; it signifies and seals our adoption into Christ, our cleansing from sin, and our commitment to belong to the Lord and to his church.
Is baptism with water the washing away of sin itself?
No, only the blood of Christ and the renewal of the Holy Spirit can cleanse us from sin.