To repent means that a person knows and confesses from the heart that, as the Scripture says, he was conceived and born in sin [Ps. 51:5] and is, therefore, by nature a child of wrath [Eph. 2:3], condemned to everlasting death and damnation, and that here all works are of no avail. They only make things go from bad to worse so long as people think they can accomplish by them what belongs to Christ alone, the sole mediator between God and men, who sacrificed Himself for us all that we might have forgiveness of sins through Him.
If you believe the former, then you have the latter. If not, you will never ever be free [of sin], even if you torture yourself to death. For it is called forgiveness of sins, not a payment for sins; a gift, not merit. But what God bestows on you out of pure grace for the sake of Christ, that is something for which you, you poor sack of maggots, cannot pay Him, nor can you buy or earn it from Him.*
*M. Luther, First Sermon at the Baptism of Bernard of Anhalt Matthew 3:1–17. In C. B. Brown (Ed.), J. S. Bruss (Trans.), Luther’s Works: Sermons V (Vol. 58, p. 39).