"Now the true meaning of Christianity is this: that a man first acknowledge, through the Law, that he is a sinner, for whom it is impossible to perform any good work. For the Law says: “You are an evil tree. Therefore everything you think, speak, or do is opposed to God. Hence you cannot deserve grace by your works. But if you try to do so, you make the bad even worse; for since you are an evil tree, you cannot produce anything except evil fruits, that is, sins. ‘For whatever does not proceed from faith is sin’ (Rom. 14:23). Trying to merit grace by preceding works, therefore, is trying to placate God with sins, which is nothing but heaping sins upon sins, making fun of God, and provoking His wrath. When a man is taught this way by the Law, he is frightened and humbled. Then he really sees the greatness of his sin and finds in himself not one spark of the love of God; thus he justifies God in His Word and confesses that he deserves death and eternal damnation. Thus the first step in Christianity is the preaching of repentance and the knowledge of oneself.
The second step is this: If you want to be saved, your salvation does not come by works; but God has sent His only Son into the world that we might live through Him. He was crucified and died for you and bore your sins in His own body (1 Peter 2:24). Here there is no “congruity” or work performed before grace, but only wrath, sin, terror, and death. Therefore the Law only shows sin, terrifies, and humbles; thus it prepares us for justification and drives us to Christ. For by His Word God has revealed to us that He wants to be a merciful Father to us. Without our merit—since, after all, we cannot merit anything—He wants to give us forgiveness of sins, righteousness, and eternal life for the sake of Christ. For God is He who dispenses His gifts freely to all, and this is the praise of His deity. But He cannot defend this deity of His against the selfrighteous people who are unwilling to accept grace and eternal life from Him freely but want to earn it by their own works. They simply want to rob Him of the glory of His deity. In order to retain it, He is compelled to send forth His Law, to terrify and crush those very hard rocks as though it were thunder and lightning."
Luther, M. (1999, c1963). Vol. 26: Luther's works, vol. 26 : Lectures on Galatians, 1535, Chapters 1-4 (J. J. Pelikan, H. C. Oswald & H. T. Lehmann, Ed.). Luther's Works (26:126). Saint Louis: Concordia Publishing House.