Behold, my brethren, how much a fast can do, and in what manner the law commands us to fast. It is required that not only with the body should we fast, but with the soul. Now the soul is humbled when it does not follow wicked opinions, but feeds on becoming virtues. For virtues and vices are the food of the soul, and it can eat either of these two meats, and incline to either of the two, according to its own will. If it is bent toward virtue, it will be nourished by virtues, by righteousness, by temperance, by meekness, by fortitude, as Paul says; 'Being nourished by the word of truth (1 Timothy 4:6.)' Such was the case with our Lord, who said, 'My meat is to do the will of My Father which is in heaven (John 4:34.)' But if it is not thus with the soul, and it inclines downwards, it is then nourished by nothing but sin. For thus the Holy Ghost, describing sinners and their food, referred to the devil when He said, 'I have given him to be meat to the people of Æthiopia. ' For this is the food of sinners. And as our Lord and Saviour Jesus Christ, being heavenly bread, is the food of the saints, according to this; 'Unless you eat My flesh, and drink My blood (John 6:53);' so is the devil the food of the impure, and of those who do nothing which is of the light, but work the deeds of darkness. Therefore, in order to withdraw and turn them from vices, He commands them to be nourished with the food of virtue; namely, humbleness of mind, lowliness to endure humiliations, the acknowledgment of God. For not only does such a fast as this obtain pardon for souls, but being kept holy, it prepares the saints, and raises them above the earth.
--St. Athanasius, Letter 1
