From Martin Luther’s Commentary on the Epistle to the Galatians (1535), trans. Theodore Graebner (Grand Rapids, Michigan: Zondervan Publishing House, 1949) p. 60
[Ch. 2] VERSE 14. But when I saw that they walked not uprightly according to the truth of the gospel.
No one except Paul had his eyes open. Consequently it was his duty to reprove Peter and his followers for swerving from the truth of the Gospel. It was no easy task for Paul to reprimand Peter. To the honor of Peter it must be said that he took the correction. No doubt, he freely acknowledged his fault.
The person who can rightly divide Law and Gospel has reason to thank God. He is a true theologian. I must confess that in times of temptation I do not always know how to do it. To divide Law and Gospel means to place the Gospel in heaven, and to keep the Law on earth; to call the righteousness of the Gospel heavenly, and the righteousness of the Law earthly; to put as much difference between the righteousness of the Gospel and that of the Law, as there is difference between day and night. If it is a question of faith or conscience, ignore the Law entirely. If it is a question of works, then lift high the lantern of works and the righteousness of the Law. If your conscience is oppressed with a sense of sin, talk to your conscience. Say: “You are now groveling in the dirt. You are now a laboring ass. Go ahead, and carry your burden. But why don’t you mount up to heaven? There the Law cannot follow you!” Leave the ass burdened with laws behind in the valley. But your conscience, let it ascend with Isaac into the mountain.
In civil life obedience to the law is severely required. In civil life Gospel, conscience, grace, remission of sins, Christ Himself, do not count, but only Moses with the lawbooks. If we bear in mind this distinction, neither Gospel nor Law shall trespass upon each other. The moment Law and sin cross into heaven, i.e., your conscience, kick them out. On the other hand, when grace wanders unto the earth, i.e., into the body, tell grace: “You have no business to be around the dreg and dung of this bodily life. You belong in heaven.”
The right separation between Law and Gospel is very important to know. Christian doctrine is impossible without it. Let all who love and fear God, diligently learn the difference, not only in theory but also in practice.
When your conscience gets into trouble, say to yourself: “There is a time to die, and a time to live; a time to learn the Law, and a time to unlearn the Law; a time to hear the Gospel, and a time to ignore the Gospel. Let the Law now depart, and let the Gospel enter, for now is the right time to hear the Gospel, and not the Law.” However, when the conflict of conscience is over and external duties must be performed, close your ears to the Gospel, and open them wide to the Law.
Are you kinda poor? Do you still want to read Walther’s gem of a book that expounds on the topic which Luther treats here? Then this one’s for you: