Anyone who can judge rightly between the law and the Gospel should thank God and know that he is a true theologian. …The way to discern the one from the other is to place the Gospel in heaven and the law on the earth, to call the righteousness of the Gospel heavenly and the righteousness of the law earthly, and to put as much difference between the righteousness of the Gospel and of the law as God has made between heaven and earth, between light and darkness, between day and night. If it is a question of faith or conscience, let us utterly exclude the law and leave it on the earth; but if we are dealing with works, let us light the lantern of works and of the righteousness of the law. The sun and light of the Gospel and grace should shine in the day, and the lantern of the law in the night.
Martin Luther. Galatians – Crossway Classic Commentaries (83). Wheaton, IL: Crossway Books.