How you treat people on Monday is the test of the authenticity of your fasting on Sunday. Fasting that leaves our daily lives unchanged in sin is the butt of Gods ridicule: Is it a fast like this . . . for bowing ones head like a reed? (iSAIAH 58 verse 5). In other words, the gestures of such fasting are no more spiritual than a bent reed in the swamp.
Woe to the fasting that leaves sin in our lives untouched.
The only authentic fasting is fasting that includes a spiritual
attack against our own sin. Is our fasting really a hunger for God? We test whether it is by whether we are hungering for our own holiness. To want God is to hate sin. For God is holy, and we cannot love God and love sin. Fasting that is not aimed at starving sin while feasting on God is self-deluded. It is not really God that we hunger for in such fasting.
The hunger of fasting is a hunger for God, and the test of that hunger is whether it includes a hunger for holiness.
Piper, Hunger for God book, p135