The law says, “do this”, and it is never done. Grace says, “believe in this”, and everything is already done.
The first part is clear from what has been stated by the Apostle and his interpreter, St. Augustine, in many places. And it has been stated often enough above that the “law works wrath” and keeps all men under the curse. The second part is clear from the same sources, for faith justifies. And the law (says St. Augustine) commands what faith obtains. For through faith Christ is in us, indeed, one with us. Christ is just and has fulfilled all the commands of God, wherefore we also fulfill everything through him since he was made ours through faith.
Martin Luther, Heidelberg Disputation