Of those who fast out of rage.
There is too another evil sort of vexation which would not be worth mentioning were it not that we know it is allowed by some of the brethren who, when they have been vexed or enraged actually abstain persistently from food, so that (a thing which we cannot mention without shame) those who when they are calm declare that they cannot possibly put off their refreshment to the sixth or at most the ninth hour, when they are filled with vexation and rage do not feel fasts even for two days, and support themselves, when exhausted by such abstinence, by a surfeit of anger. Wherein they are plainly guilty of the sin of sacrilege, as out of the devils own rage they endure fasts which ought specially to be offered to God alone out of desire for humiliation of heart and purification from sin: which is much the same as if they were to offer prayers and sacrifices not to God but to devils, and so be worthy of hearing this rebuke of Moses: “They sacrificed to devils and not to God; to gods whom they knew not.” 1996
Deut. xxxii. 17.