Heidelberg Catechism: Lord's Day 26

Q: 69. How are you admonished and assured by holy baptism, that the one sacrifice of Christ upon the cross is of real advantage to you?
A: Thus: That Christ appointed this external washing with water, adding thereto this promise, that I am as certainly washed by his blood and Spirit from all the pollution of my soul, that is, from all my sins, as I am washed externally with water, by which the filthiness of the body is commonly washed away.

Q: 70. What is it to be washed with the blood and Spirit of Christ?
A: It is to receive of God the remission of sins, freely, for the sake of Christ's blood, which he shed for us by his sacrifice upon the cross; and also to be renewed by the Holy Spirit, and sanctified to be members of Christ, that so we may more and more die unto sin, and lead holy and blameless lives.

Q: 71. Where has Christ promised us, that he will as certainly wash us by his blood and Spirit, as we are washed with the water of baptism?
A: In the institution of baptism, which is thus expressed: "Go ye, therefore, and teach all nations, baptizing them in the name of the Father, and of the Son, and of the Holy Spirit", And "he that believeth, and is baptized, shall be saved; but he that believes not, shall be damned.", This promise is also repeated, where the scripture calls baptism "the washing of regenerations" and the washing away of sins.

Applications
1. Notice the first question. How do we know that the cross is of real advantage to us? We all wonder this at times. How do I know that I am a Christian or that Christ died for me? One way is by looking at our baptisms and the promise given there. As sure as the water was poured, sprinkled or I was dunked in it, I can be assured that Christ shed his blood for me.

2. The answer to the second perfectly balances justification and sanctification. Christ's blood means that my sins are forgiven and that I should lead a holy life. It does not mean that my sins are forgiven so I can do as I please. It does not mean that I must earn forgiveness. It means that I am holy in Christ and I need to grow up into that holiness.