From the Shorter Catechism

Q. 94. What is Baptism?

A. Baptism is a Sacrament, wherein the washing with water, in the name of the Father, and of the Son, and of the Holy Ghost, doth signify and seal our ingrafting into Christ, and partaking of the benefits of the covenant of grace, and our engagement to be the Lord’s. Matt. 28:19; Rom. 6:4; Gal. 3:27.