In a letter to a man that had asked whether the sins of believers shall be publicly declared at the Great Day, Newton responded in part:
I think those are the sweetest moments in this life, when we have the clearest sense of our own sins, provided the sense of our acceptance in the Beloved is proportionally clear, and we feel the consolations of his love, notwithstanding all our transgressions. When we arrive in glory, unbelief and fear will cease forever: our nearness to God, and communion with him, will be unspeakably beyond what we can now conceive. Therefore the remembrance of our sins will be no abatement of our bliss, but rather the contrary.
You can read the entire letter here.
I think Newton gets this idea from the sinful woman in Luke 7:36-50. The more we come to grips with our sinfulness, the more they too become a cause for rejoicing. We have been forgiven much; few people understood this as deeply as John Newton. May the Lord cause us to rejoice that even the darkest of sins cannot separate us from His love.