The Heidelberg Catechism: Lord's Day 26
69. Question: How does holy baptism signify and seal to you that the one
sacrifice of Christ on the cross benefits you?
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 Spirit wash away the impurity of my soul, that is, all my sins.
 Matt. 28:19.  Matt. 3:11; Mark 16:16; John 1:33; Acts 2:38; Rom. 6:3, 4; I Pet. 3:21.
70. Question: What does it mean to be washed with Christ's blood and Spirit?
Answer: To be washed with Christ's blood means to receive forgiveness of sins from God, through grace, because of Christ's blood, poured out for us in His sacrifice on the cross. To be washed with His Spirit
means to be renewed by the Holy Spirit and sanctified to be members of Christ, so that more and more we become dead to sin and lead a holy and blameless life.
 Ez. 36:25; Zech. 13:1; Eph. 1:7; Heb. 12:24; I Pet. 1:2; Rev. 1:5; 7:14.  John 3:5-8; Rom. 6:4; I Cor. 6:11; Col. 2:11, 12.
71. Question: Where has Christ promised that He will wash us 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: Go therefore and make disciples of all nations, baptizing them in the name of the Father and of the Son and of the Holy Spirit (Matthew 28:19). He who believes and is baptized will be saved, but he who does not believe will be condemned (Mark 16:16). This promise is repeated
where Scripture calls baptism the washing of regeneration and the washing away of sins (Titus 3:5; Acts 22:16).
The Heidelberg Catechism
Lord's Day 1