What is at issue in fasting is not that one fasts, but that one does it willingly. And what benefit is fasting for you, since you undertake the fast unwillingly? Fasting is the singular plow of holiness: it tills the heart, it eradicates sins, it uproots offenses, it stamps out vices, it sows charity, it nourishes the crop, and it furnishes a harvest of innocence.
Therefore, Christ's disciples, completely and firmly placed in the field of holiness and gathering handfuls of virtues, now providing a new kind of bread, are unable to engage in the old kind of fasts that are bragged about verbally, promoted with a pallid complexion, advertised by disfigurement, and pleasing to human eyes, but not to divine ones.
Sermon 31, section 2.