Jesus blasted the religious elite for scrubbing up their persona/image but not tending to their soul. He compares them to cleaning the outside of the cup and leaving the inside full of "greed and self indulgence".
But in his short metaphor (cup cleaning 101) he shares the sweet benefits of scrubbing from the inside out. When the soul is surrendered, and tended to by God, it not only cleans our soul...it gives our lives a radiance we could never produce with all our religious elbow grease.
How great is God?! His work is so complete and thorough that it cleanses the nastiest of stains, even those residual effects of years of bad choices. This is great news for anyone who feels like they can't measure-up and are exhausted from working hard to look right in the eyes of others.
Rest my friend. Surrender to God the ugliest inner parts and the secrets of your soul. Soon you'll shine like the Son.