This temptation is more necessary to you than food and drink. Let me remind you what happened to me when I was about your age. When I first entered the monastery it came to pass that I was sad and downcast, nor could I lay aside my melancholy. On this account I made confession to and took counsel with Dr. Staupitz and opened to him what horrible and terrible thoughts I had. Then said he: 'Don't you know, Martin, that this temptation is useful and necessary to you? God does not exercise you thus without reason. You will see that he intends to use you as his servant to accomplish great things.' And so it turned out. . . . although at the time when I suffered this temptation I never would have believed it possible. . . .
Whenever the devil pesters you . . . at once seek out the company of men, drink more, joke and jest, or engage in some other form of merriment. Sometimes it is necessary to drink a little more, play, jest, or even commit some sin in defiance and contempt of the devil in order not to give him an opportunity to make us scrupulous about trifles. . . .
When the devil throws our sins up to us and declares that we deserve death and hell, we ought to speak thus: 'I admit that I deserve death and hell. What of it? Does this mean that I shall be sentenced to eternal damnation? By no means. For I know One who suffered and made satisfaction in my behalf. His name is Jesus Christ, the Son of God. Where he is, there I shall be also.
--quoted in Richard Marius, Martin Luther: The Christian between God and Death (Harvard University Press 2000), 122-23
A cold, sluggish heart will take this as encouragement to sin. One way this is mitigated is by remembering the kind of person to whom this letter was directed. Fundamentally, though: a heart sensitized to the gospel and to our own startling ongoing sin even as those who have been born again will find, with me, a world of gospel wisdom and sanity here.