The essence of sin is our attempt to take the place of God; the essence of the Christian faith is God taking our place.
Chandler, Matt; Geiger, Eric; Patterson, Josh (2013-11-26). Creature of the Word: The Jesus-Centered Church (Kindle Locations 2209-2210). B&H Publishing Group. Kindle Edition.
If this is correct, then legalistic, law driven obedience is sin, for it seeks to put the burden of our salvation on our own shoulders. In essence, it is us attempting to take the place of God. That burden has already been taken up by Christ. He took our place. He did what I could not and cannot do, in that He lived a perfect life and then sacrificed it on my behalf. How do you live up to that?
You don't live up to it, but you do respond to it. And in that is beauty.