The prophet [Isaiah] said of God, ‘You have put all my sins behind your back.’ When something is behind our back, it is out of sight. We can’t see it anymore. God says He has done that with our sins. It is not that we haven’t sinned or, as Christians, do not continue to sin. We know we sin daily — in fact, many times a day. Even as Christians our best efforts are still marred with imperfect performance and impure motives. But God no longer ‘sees’ either our deliberate disobedience or our marred performances. Instead He ‘sees’ the righteousness of Christ, which He has already imputed to us.
Does this mean God ignores our sins like an overindulgent, permissive father who lets his children grow up undisciplined and ill-behaved? Not at all. In His relationship to us as our Heavenly Father, God does deal with our sins, but only in such a way as for our good. He does not deal with us as our sins deserve, which would be punishment, but as His grace provides, which is for our good.
— Jerry Bridges