[The following paragraphs are taken from Luther’s 1535 Commentary on Galatians 2:14]

“Therefore whoever knows well how to distinguish the Gospel from the Law should give thanks to God and know that he is a real theologian. I admit that in the time of temptation I myself do not know how to do this as I should. The way to distinguish the one from the other is to locate the Gospel in heaven and the Law on earth, to call the righteousness of the Gospel heavenly and divine and the righteousness of the Law earthly and human, and to distinguish as sharply between the righteousness of the Gospel and that of the Law as God distinguishes between heaven and earth or between light and darkness or between day and night. Let the one be like the light and the day, and the other like the darkness and the night. If we could only put an even greater distance between them! Therefore if the issue is faith, heavenly righteousness, or conscience, let us leave the Law out of consideration altogether and let it remain on the earth. But if the issue is works, then let us light the lamp of works and of the righteousness of the Law in the night. So let the sun and the immense light of the Gospel and of grace shine in the day, and let the lamp of the Law shine in the night….

“In society, on the other hand, obedience to the Law must be strictly required. There let nothing be known about the Gospel, conscience, grace, the forgiveness of sins, heavenly righteousness, or Christ Himself; but let there be knowledge only of Moses, of the Law and its works. When these two topics, the Law and the Gospel, are separated this way, both will remain within their limits. The Law will remain outside heaven, that is, outside the heart and the conscience; and, on the other hand, the freedom of the Gospel will remain outside the earth, that is, outside the body and its members. And just as soon as the Law and sin come into heaven, that is, into the conscience, they should be promptly ejected. For then the conscience should know nothing about the Law and sin but should know only about Christ. On the other hand, when grace and freedom come into the earth, that is, into the body, you must say: “You have no business here among the dirt and filth of this physical life. You belong in heaven!”

“Peter had confused this distinction between the Law and the Gospel, and thus he had persuaded the believers that they had to be justified by the Gospel and the Law together. This Paul refused to tolerate. Therefore he rebuked Peter. He did not want to put him to shame, but he wanted to separate these two very sharply again, namely, that the Law justifies on earth and the Gospel in heaven. But the pope has not only confused the Law with the Gospel; but he has changed the Gospel into mere laws, and ceremonial laws at that. He has also confused secular matters and church matters, which is really a satanic and infernal confusion.

“The knowledge of this topic, the distinction between the Law and the Gospel, is necessary to the highest degree; for it contains a summary of all Christian doctrine. Therefore let everyone learn diligently how to distinguish the Law from the Gospel, not only in words but in feeling and in experience; that is, let him distinguish well between these two in his heart and in his conscience. For so far as the words are concerned, the distinction is easy. But when it comes to experience, you will find the Gospel a rare guest but the Law a constant guest in your conscience, which is habituated to the Law and the sense of sin; reason, too, supports this sense” (LW 26:116-117).