You cannot produce anyone in all the world to whom the title “doer of the Law” applies apart from the promise of the Gospel. “Doer of the Law” is, therefore, a fictitious term, which no one understand unless he is outside and beyond the Law in the blessings and faith of Abraham. Thus he is a true doer of the Law who receives the Holy Spirit through faith in Christ and then begins to love God and to do good to his neighbor. Hence “to do” includes faith at the same time. Faith takes the doer himself and makes him into a tree, and his deeds become fruit. First there must be a tree, then the fruit. For apples do not make a tree, but a tree makes apples. So faith first makes the person, who afterwards performs works. To keep the Law without faith, therefore, is to make apples without a tree, out of wood or mud, which is not to make apples but to make mere phantasies. But once the tree has been planted, that is, once there is the person or doer who comes into being through faith in Christ, then works follow. For there must be a doer before deeds, not deeds before the doer. So “the doer of the Law is justified”: that is, he is accounted as righteous (Rom. 2:13).
Luther’s Works, volume 26 (255-256)