"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 [or 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. It suffices that through God’s glory we have recognized the Lamb who takes away the sin of the world. No sin can separate us from Him, even if we were to kill or commit adultery thousands of times each day. Do you think such an exalted Lamb paid merely a small price with a meager sacrifice for our sins? Pray hard for you are quite a sinner."
I won't lie, Luther has made some strange statements before, so I am not surprised about this quote. I have a hard time reading his commentaries.
It sounds to me that Luther is using a hyperbole about the old nature that hangs on and still tempts us, and that despite our daily failings, we must nonetheless hold stronger to Christ. Nobody accidently commits sin, but even so, let us boldly come to Christ all the more.