Let us remember this [the righteousness apart from works] well in our personal trials, when the devil accuses and terrifies our conscience to bring it to the point of despair. He is the father of lies (Jn 8:44) and the enemy of Christian freedom. At every single moment, therefore, he torments us with false terrors, so that when this freedom has been lost, the conscience is in continual fear and feels guilt and terrors.
I say, when that great dragon, the ancient serpent, the devil, the deceiver of the whole world, who accuses our brethren day and night before God (Rev 12:9-10) comes to you and accuses you not only of failing to do anything good but of transgressing against the law of God, say: 'You are troubling me with the memory of past sins; in addition, you are telling me that I have not done anything good. This means nothing to me. For if I either trusted in my performance of good works or despaired because I failed to perform them, in either case Christ would be of no avail to me. Therefore whether you place before me my sins or on my good works, I do not care; for I put both of them out of sight and lean solely upon the freedom for which Christ has set me free. This I know to be useful to me! Therefore I shall not render Him useless to me, which is what would happen if I either presumed that I shall attain grace and eternal life because of my good works or despaired of my salvation because of my sins."
--Martin Luther, Commentary on Galatians