We must know what the law is, and what the gospel is. The law commands and requires us to do certain things. The law is thus directed solely to our behavior and consists in making requirements. For God speaks through the law, saying, “Do this, avoid that, this is what I expect of you.” The gospel, however, does not preach what we are to do or to avoid. It sets up no requirements but reverses the approach of the law, does the very opposite, and says, “This is what God has done for you; he has let his Son be made flesh for you, has let him be put to death for your sake.” So, then, there are two kinds of doctrine and two kinds of works, those of God and those of men. Just as we and God are separated from one another, so also these two doctrines are widely separated from one another. For the gospel teaches exclusively what has been given us by God, and not—as in the case of the law—what we are to do and give to God.
Martin Luther, “How Christians Should Regard Moses”, LW 35:162.
…it is so important to distinguish the two words [Law and Gospel] properly and not mingle them together. Otherwise you will not be able to have or hold on to a correct understanding of either of them. Instead, just when you think you have them both, you will have neither.
Martin Luther, Sermon for the Circumcision of Our Lord, 1532, WA 36:9:28ff. Translated by Willard L. Bruce in Concordia Journal, April, 1992, p. 153.
A few times—when I did not bear this principal teaching in mind—the devil caught up with me and plagued me with Scripture passages until heaven and earth became too small for me. Then all the works and laws of man were right, and not an error was to be found in the whole papacy. In short, the only one who had ever erred was Luther. All my best works, teaching, sermons, and books had to be condemned. The abominable Mohammed almost became my prophet, and both Turks and Jews were on the way to pure sainthood…. If by choice or of necessity you must deal with matters concerning the law, works, sayings, and examples of the fathers, then remember first of all to keep this principal teaching before you, and do not be caught without it, so that the dear sun of Christ will shine in your heart. Then you can freely and safely discuss and discriminate in all laws, examples, sayings, and works.
Martin Luther, “Exposition of Psalm 117”, LW 14:37-38.
I admit, of course, that there are many texts in the Scriptures that are obscure and abstruse, not because of the majesty of their subject matter, but because of our ignorance of their vocabulary and grammar; but these texts in no way hinder a knowledge of all the subject matter of Scripture. For what still sublimer thing can remain hidden in the Scriptures, now that the seals have been broken, the stone rolled from the door of the sepulcher [Matthew 27:66; 28:2], and the supreme mystery brought to light, namely, that Christ the Son of God has been made man, that God is three and one, that Christ has suffered for us and is to reign eternally? Are not these things known and sung even in the highways and byways? Take Christ out of the Scriptures, and what will you find left in them?
Martin Luther, “The Bondage of the Will”, LW 33:25-26.