I've been asked whether or not it really matters that Jesus gave us a thoroughly good and pure heart when he rescued us. After all, isn't it enough to know that he lives in me, and that his goodness dwells in me? Why do I need to be thought of as having a good and noble heart now?
No. First, Jesus won't simply be good for you [on your behalf]. His desire was to restore your heart so that it possessed the same purity and goodness his does. You can't mature as an individual, unique person if you don't possess a goodness that is now yours. Yes, that goodness comes solely from Christ's work in you and for you; but nevertheless, just as I need to allow my own children to grow up in the strength I nourish in them -- so that it becomes their strength -- so Jesus must give us a new heart so that it becomes our strength. He no longer wishes us to merely borrow his goodness.
Why would he merely live in you, without dramatically changing the core of your being first - without addressing the problem of the heart itself. As goes the heart, so goes the person. He had to first clean the inside of the cup.
Jesus intent has always been to rescue the heart. You can even see this in the Prophets' own writing. Ezekiel 36:26 even predicts Jesus's restoring work of the heart.
THE RUINED HEART:
THE RESTORED HEART: