And he's a lifesaver (for me, anyway). No classic writer outside the Bible has helped me more with gospel/law, grace/sin. And Luther is so helpful because this wasn't theoretical for him. He felt the weight of his own sin and clung desperately to the cross of Christ, his sole hope in the despair of his depravity. He felt this stuff in his bones. Take comfort in Luther's preaching, if you are brokenhearted and tempted to doubt your assurance.

“Faith, if it is to be sure and steadfast, must lay hold upon nothing else but Christ alone, and in the conflict and terrors of conscience it has nothing else to lean on but this precious pearl Christ Jesus. So, he who apprehends Christ by faith, although he be terrified with the law and oppressed with the weight of his sins, yet he may be bold to glory that he is righteous. How? Even by that precious jewel Christ Jesus, whom he possesses by faith.”

-- from Luther's Commentary on Galatians