"If you are a preacher of mercy, do not preach an imaginary but the true mercy. If the mercy is true, you must therefore bear the true, not an imaginary sin. God does not save those who are only imaginary sinners. Be a sinner, and let your sins be strong (sin boldly), but let your trust in Christ be stronger, and rejoice in Christ who is the victor over sin, death, and the world. We will commit sins while we are here, for this life is not a place where justice resides. We, however, says Peter (2. Peter 3:13) are looking forward to a new heaven and a new earth where justice will reign."
Letter 99, Paragraph 13. Erika Bullmann Flores, Tr. from: Dr. Martin Luther's Saemmtliche Schriften Dr. Johann Georg Walch Ed. (St. Louis: Concordia Publishing House, N.D.), Vol. 15, cols. 2585-2590. 
Hat Tip: http://en.wikiquote.org/wiki/Martin_Luther