How does holy baptism instruct and assure you that Christ’s one sacrifice on the cross benefits you?
Thus: that Christ appointed this external washing with water, adding this promise, that I am as certainly washed by His blood and Spirit from all the defilement of my soul, that is from all my sins, as I am washed externally with water by which the dirt of the body is commonly washed away.
Scripture references
What is it to be washed with the blood and Spirit of Christ?
It is to receive of God the forgiveness 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 to sin and lead holy and blameless lives.
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?
In the institution of baptism which is expressed in these words: Go therefore and teach all nations, baptising them in the name of the Father and of the Son and of the Holy Spirit. And the one who believes and is baptised shall be saved, but the one who does not believe will be condemned. This promise is also repeated where Scripture calls baptism the washing of regeneration and the washing away of sins.