The Fight of Faith
I remember how Doctor Staupitz used to say to me: "I have promised God a thousand times that I would become a better man, but I never kept my promise. From now on I am not going to make more vows. Experience has taught me that I cannot keep them. Unless God is merciful to me for Christ's sake and grants unto me blessed departure, I shall not be able to stand before Him." His was a God-pleasing despair. No true believer trusts in his own righteousness, but says with David, "Enter not into judgment with your servant, for in your sight no man living will be justified." (Ps. 143.2) Again, "If you, Lord, should mark iniquities, O Lord, who would stand?" (Ps. 130:3)
No man is to despair of salvation just because he is aware of the lust of the flesh. Let him be aware of it so long as he does not yield to it. The passion of lust, wrath, and other vices may shake him, but they are not to get him down. Sin may assail him, but he is not to welcome it. Yes, the better Christian a man is, the more he will experience the heat of conflict.
(Luther: Commentary on the Epistle to the Galatians)