Yes, All your sins re forgiven. No argument there. But if I break your window, and all my sins are forgiven, would you agree that I owe you a window and maybe an apology? If I break your window, and I'm having a good day (one with Guiness instead of ice-water, for example) I WANT to replace your window, either myself or by paying somebody else to do it.
This is NOT meant as a metaphor. I'm totally forgiven. But I think many, including the person picking up shards of glass from the carpet would think that that forgiveness (a) doesn't wipe out every debt, (b) actually prompts me to pay my debts.
So if you're with me so far, where does that fit in to the whole schema?
Good question. The answer is: Jesus paid for your window before you broke it. Yes, you should be contrite, and offer an apology, and try not to break another one, but the cost and punishment for the window is already paid. To try to pay for a window that Jesus already paid for is to say to Jesus, “Your payment was not enough, I have to contribute, also.” This is not the way with God. Jesus paid for all the windows you will ever break.