[E]ven though we are now in faith . . . the heart is always ready to boast of itself before God and say: 'After all, I have preached so long and lived so well and done so much, surely he will take this into account.' We even want to haggle with God to make him regard our life. . . .--LW 51:284 (HT: Jean Larroux)
But it cannot be done. With men you may boast: I have done the best I could toward everyone, and if anything is lacking I will still try to make recompense. But when you come before God, leave all that boasting at home and remember to appeal from justice to grace. Let anybody try this and he will see and experience how exceedingly hard and bitter a thing it is for a man, who all his life has been mired in his work righteousness, to pull himself out of it and with all his heart rise up through faith in the one Mediator.
I myself have now been preaching and cultivating it through reading and writing for almost twenty years and still I feel the old clinging dirt of wanting to deal so with God that I may contribute something, so that he will have to give me his grace in exchange for my holiness. Still I cannot get it into my head that I should surrender myself completely to sheer grace; yet this is what I should and must do.
See Steven Ozment's outstanding comments on this passage from Luther in The Age of Reform (1250-1550): An Intellectual and Religious History of Late Medieval and Reformation Europe (Yale University Press, 1980), 375-77