Human beings have not kept the law of God but have transgressed it. Their corrupted human nature, thoughts, words, and deeds battle against the law. For this reason they are subject to God’s wrath, to death and all temporal afflictions, and to the punishment of the fires of hell. As a result, the gospel in its strict sense teaches what people should believe, namely, that they receive from God the forgiveness of sins; that is, that the Son of God, our Lord Christ, has taken upon himself the curse of the law and borne it, atoned and paid for all our sins; that through him alone we are restored to God’s grace, obtain the forgiveness of sins through faith, and are delivered from death and all the punishments of our sins and are saved eternally.
For everything that provides comfort — everything that offers the favor and grace of God to those who have transgressed the law — is and is called the gospel in the strict sense. It is good news, joyous news, that God does not want to punish sin but to forgive it for Christ’s sake.
Accordingly, all repentant sinners should believe in, that is, place their trust alone in, the Lord Christ, who ‘was handed over to death for our trespasses and was raised for our justification.’ (Rom. 4:25)
From: the Solid Declaration, Article V: Law and Gospel, in The Book of Concord (Editors: Kolb/Wengret, p. 585, paragraphs 20-22)