Whenever happy confidence in the sovereign power of God for our own lives and the lives of others grows weak, legalism creeps in. We inevitably try to compensate for loss of dynamic faith by increased moral resolve and the addition of man-made regulations. But wherever joyful confidence in the power of God is waning, the flesh is waxing. Which means that the morality we had hoped would save ourselves and the regulations we hoped would purify our church fall victim to the massive power of the flesh and become its instruments of self-reliance and self-sufficiency.
John Piper