I once read a story about Martin Luther and a friend of his. They were writing to one another and Luther's friend had some concerns about whether or not he was trusting Christ enough. The story goes like this:
Martin Luther was translating the Bible into German at the Wartburg castle and could only have contact with his colligue Phillip Melanchthon by courier. Melanchthan had a different sort of temperament than Luther. Some would call him timid; others of a less generous bent might call him spineless.
At one time, while Luther was off in the Wartburg castle translating, Melanchthon had another one of his attacks of timidity. He wrote to Luther, "I woke this morning wondering if I trust Christ enough." Luther received such letters from him regularly. He had a tendency to naval-gaze and to wonder about the state of his inner faith, and whether it was enough to save. Finally, in an effort to pull out all the stops and pull Melanchthon out of himself, Luther wrote back and said, "Melanchthon! Go sin bravely! Then go the cross and bravely confess it! The whole gospel is outside of us."
A couple things to point out here:
1) Luther was not encouraging sin. If you read the account carefully, Melanchthon often had these feelings of doubt about his state before God. Luther was just responding to him based upon what he knew about his friend. And it did seem that he was focusing too much on his inner faith. So please do not take this account to mean Christians are supposed to not care about their own sin. Of course that's not true. But when we do sin, because of the work of Christ, we can and should go to the cross boldly.
2) Luther was encouraging gospel confidence. I know from my own Christian life that two or three years ago I would've looked a lot like Melanchthon. I often looked to myself for assurance of my salvation. In other words, even though I knew Christ paid it all on the cross, I still had second thoughts about my state before God. I was somehow undermining the work of Christ by clinging to my own supposed righteousness. I had forgotten a simple truth that Luther rightfully stated, "The whole gospel is outside of us." In other words, even after we become Christians it is still by grace alone! Any progress that we make in obedience to God's commands is still by grace alone! And the great result of this mindset is that, in the end, we will end up living a more obedient Christian life.
Even today I greatly struggle with this truth that my acceptance before God is based solely upon the work of Christ alone. Yet keeping this truth in mind we can live in great freedom knowing our standing before God is perfect because of our Savior Jesus! We are free.