That the Law is something entirely different from the promise is plain. The Law thunders: “Thou shalt, thou shalt not.” The promise of the “seed” (Christ) pleads: “Take this gift of God.” If the inheritance of the gifts of God were obtained by the Law, God would be a liar. We would have the right to ask Him: “Why did you make this promise in the first place: ‘In thy seed shall all the nations of the earth be blessed’? Why did you not say: ‘In thy works thou shalt be blessed’?”
Martin Luther, Commentary on Galatians
Law, Grace, & Daily Living 