The Institutes, Book 5
Bodily Fasting for the Sake of Purity of Heart
And so we know that the reason we ought to give attention to bodily abstinence is that by this fasting we may come to purity of heart. Otherwise our labors will be spent in vain, if we endure this tirelessly with the goal in view but cannot reach the end for which we have endured such trials. It would have been better to abstain from the forbidden foods of the soul than to fast in the body from things that are indifferent and harmless. For in the case of these there is a simple and harmless receiving of something God created, which has nothing wrong in itself. But in the case of the former there is from the very first a dangerous tendency to devour our brothers, of which it is said, “Do not love slander, or you will be uprooted.” And about anger and jealousy blessed Job says, “For anger kills the fool, and envy slays the child.” At the same time it should be noticed that the one who is angry is set down as a fool, and the one who is jealous as a child. The first is rightly considered a fool, since of his own accord he brings death on himself, goaded by the stings of anger. The second, by his envy, proves that he is a child and the lesser one. For while he envies another, he shows that the one at whose prosperity he is vexed is greater than he.