Excuse me but comparing David or any of the Old Testament prophets is absurd. Show me where any of them advocating sinning as much as possible.
Blessed Father Luther? I think not.
Only that wasn't what Luther was doing. Are you familiar with the term "hyperbole"? It was and still is a way to use exaggeration to make a rhetorical point. You may have used it from time to time, even (i.e.; I'm so hungry I could eat a horse.) That was the context of Luther's talking about sin. This from http://beggarsallreformation.blogspot.com/search?q=sin+boldly:
Luther was prone to strong hyperbole. It's his style, and this statement is a perfect example. The first thing to recognize is that the sentence is a statement of comparison. Luther's point is not to go out and commit multiple amounts of gleeful sin everyday, but rather to believe and rejoice in Christ even more boldly despite the sin in our lives. Christians have a real savior. No amount of sin is too much to be atoned for by a perfect savior whose righteousness is imputed to the sinner who reaches out in faith.
See: Luther: Be a sinner, and let your sins be strong And: A Look at Justification By Faith Alone and Good Works in Luthers Theology. For an anthology of statements from Luther on a living faith producing works, see: Quotations From Luther on Faith And Works. "Faith," said Luther, "is a living,restless thing. It cannot be inoperative. We are not saved by works; but if there be no works, there must be something amiss with faith."