In explaining the difference between the law and the gospel, Martin Luther stated:
"Now the first sermon (10 commandments), 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 behaviour 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."