Thanks to Amy Boucher Pye in her book The Living Cross for bringing this verse to my attention. The whole verse is powerful, isn’t it? Sermons, if not books, could be written about that first clause. But it was the underlined portion that stood out. 
We often struggle to let go of the guilt of past sins, feeling that God must be as acutely aware of them as we are. We can think he is waiting for us to slip up again, and withholding his full forgiveness until we can prove that we’re not going to repeat the same errors. 

But this imagery assures us that that is not the case. He has cast all our sins behind his back. The NIV says ‘put’, which is good enough, but ‘cast’ is even better. I picture him screwing them up and throwing them over his shoulder, like a writer with a badly-written page. All he sees is the clean sheet in front of him. Fresh and clear and ready to write on again. 

Each new sin is dealt with in the same way – I think I’m theologically correct in saying that even before we come to him in repentance the sin is scrunched up and cast away. That is my understanding of what it means to be washed by the blood of the Lamb – that all our sins, past, present and future, are removed from us as far as the East is from the West. We confess and repent so as not to create a wall of guilt in our relationship with God, but the sin itself has been dealt with, once and for all. 

This is a great mystery, but what a wonderful gift.