Legalism and Freedom and Obedience from the heart:

"Jesus did not confront oppressive legalism by advocating license." (Jerry Sittser, The Will of God as a way of Life, p. 61)
That leads to some interesting reflection...

While Jesus sets us free from the yoke of legalism he never gave way to license. While avoiding external performance as a way of righteousness he never lowered the standard of holiness. If anything Jesus taught that the standards of the legalists were too low. He taught that the real issue was obedience from the heart and that such obedience yielded more, not less, holiness.

If we seek to use grace as a cover for license then we're only thinking about it from the head level and not the heart level. Obedience from the heart seeks to know the heart of God on any given matter; it puts the law of love above the liberty of license.