Before returning to our series of posts quoting non-Lutherans on Law and Gospel, I thought it would be helpful for all, but especially non-Lutheran readers, to hear from Luther himself on the topic. The following extract is from a sermon that was also, if I'm not mistaken, appended to copies of the Luther Bibel. The painting is 'Gesetz und Gnade' (Law and Grace), a visual depiction of the Lutheran doctrine, by Lucas Cranach the Elder. Notice how on the side depicting Law, Christ appears as Judge coming on the clouds, while in the side depicting Grace, He appears as Saviour and Redeemer.
"Dear friends, you have often heard that there has never been a public sermon from heaven except twice. Apart from them God has spoken many times through and with men on earth, as in the case of the holy patriarchs Adam, Noah, Abraham, Isaac, Jacob, and others, down to Moses. But in none of these cases did he speak with such glorious splendor, visible reality, or public cry and exclamation as he did on those two occasions. Rather God illuminated their heart within and spoke through their mouth, as Luke indicates in the first chapter of his gospel where he says, "As he spoke by the mouth of his holy prophets from of old" [Luke 1:70].
Now the first sermon is in Exodus 19 and 20; by it God caused himself to be heard from heaven with great splendor and might. For the people of Israel heard the trumpets and the voice of God himself.
In the second place God delivered a public sermon through the Holy Spirit on Pentecost [Acts 2:2-4]. On that occasion the Holy Spirit came with great splendor and visible impressiveness, such that there came from heaven the sudden rushing of a mighty wind, and it filled the entire house where the apostles were sitting. And there appeared to them tongues as of fire, distributed and resting on each of them. And they were all filled with the Holy Spirit and began to preach and speak in other tongues. This happened with great splendor and glorious might, so that thereafter the apostles preached so powerfully that the sermons which we hear in the world today are hardly a shadow compared to theirs, so far as the visible splendor and substance of their sermons is concerned. For the apostles spoke in all sorts of languages, performed great miracles, etc. Yet through our preachers today the Holy Spirit does not cause himself to be either heard or seen; nothing is coming down openly from heaven. This is why I have said that there are only two such special and public sermons which have been seen and heard from heaven. To be sure, God spoke also to Christ from heaven, when he was baptized in the Jordan [Matt. 3:17], and [at the Transfiguration] on Mount Tabor [Matt. 17:5]. However none of this took place in the presence of the general public.
God wanted to send that second sermon into the world, for it had earlier been announced by the mouth and in the books of the holy prophets. He will no longer speak that way publicly through sermons. Instead, in the third place, he will come in person with divine glory, so that all creatures will tremble and quake before him [Luke 21:25-27]; and then he will no longer preach to them, but they will see and handle him himself [Luke 24:39].
Now the first sermon, and doctrine, is the law of God. The second is the gospel. These two sermons are not the same. Therefore we must have a good grasp of the matter in order to know how to differentiate between them. 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."
From the sermon, How Christians Should Regard Moses, August 27, 1525