"Baptism is a central component of the Great Commission because baptism marks us out as a follower of Christ and assures us that by faith we have been forgiven by God. There's a lot that can be said about the meaning of baptism. It signifies our dying and rising with Christ. It signifies our sprinkling with the blood of Christ. It signifies our union with Christ. All of this is true and worth our comtemplation. But at the most basic level, the water of baptism reminds us that our sins have been washed away. All our stains have been wiped clean by the sin-scrubbing detergent of God's grace." Kevin DeYoung. The Good News We Almost Forgot. 127.

Question 69: How does baptism remind you and assure you that Christ's one sacrifice on the cross is for you personally?

Answer: In this way: Christ instituted this outward washing and with it gave the promise that, as surely as water washes away the dirt from the body, so certainly His blood and His Spirit wash away my soul's impurity, in other words, all my sins.

Question 70: What does it mean to be washed with Christ's blood and Spirit?

Answer: To be washed with Christ's blood means that God, by grace, has forgiven my sins because of Christ's blood poured our for me in His sacrifice on the cross. To be washed with Christ's Spirit means that the Holy Spirit has renewed me and set me apart to be a member of Christ so that more and more I become dead to sin and increasingly live a holy and blameless life. 

Question 71: Where does Christ promise that we are washed with His blood and Spirit as surely as we are washed with the water of baptism?

Answer: In the institution of baptism where He says: "Therefore go and make disciples of all nations, baptizing them in the name of the Father and of the Son and of the Holy Spirit." "Whoever believes and is baptized will be saved, but whoever does not believe will be condemned." This promise is repeated when Scripture calls baptism the washing of rebirth and the washing away of sins.