I found the explanation I was looking for. See the comments section below.
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, 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.
One of our pastors recently talked about this letter from Luther to Melancthon in Bible Study, in fact during our study of the Lutheran Confessions. This letter is the source of the oft wrongly quoted line "let your sins be strong". If I remember correctly, Luther wrote this in response to a specific concern/question from Melancthon. I hope that a viewer can fill me in on Melancthon's concern; I remember that it was key in understanding Luther's response. I will also email my pastor.
Luther to Melancthon, 8-1-1541