We can all see that this book was written so that we can see that we have life in God. We can all see that the author is comparing life hiding in fear, and the peace that God offers. We can all see that we should not doubt God, but believe. These are all common topics that get preached on all the time in the church. The unpredictable plot twist that I see is one that I think we as Lutherans often just want to read over.
Sin is the willful breaking of the law, or rules of conduct.
Death is the end or destruction. Sin and death then, Biblically speaking, would be our breaking the rules of conduct of the Bible resulting in our destruction. Pastor Loy retaining our sins would be an attempt to retain our destruction. I do not think this is what Jesus meant. Some disciples are locked in a room for fear of the Jews.