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St. Athanasius on Fasting

The holy Psalmist commanded, saying, “Blow with the trumpet in the new moon, on the solemn day of your feast”.... “if ye shall go out to battle in your land against your enemies that rise up against you, then ye shall proclaim with the trumpets, and shall be remembered before the Lord, and be delivered from your enemies.” Not only in wars did they blow the trumpet, but under the law, there was a festal trumpet also. Hear him again, going on to say, “And in the day of your gladness, and in your feasts, and your new moons, ye shall blow with the trumpets”. ... Now these things were typical, and done as in a shadow. But let us pass on to the meaning, and henceforth leaving the figure at a distance, come to the truth, and look upon the priestly trumpets of our Saviour, which cry out, and call us, at one time to war, as the blessed Paul saith;

“We wrestle not with flesh and blood, but with principalities, with powers, with the rulers of this dark world. with wicked spirits in heaven”. At another time the call is made to virginity, and self-denial, and conjugal harmony, saying, To virgins, the things of virgins; and to those who love the way of abstinence, the things of abstinence; and to those who are married, the things of an honourable marriage; thus assigning to each its own virtues and an honourable recompense. Sometimes the call is made to fasting, ...  for he saith, “Blow ye the trumpet in Sion: sanctify a fast”. This is a warning trumpet, and commands with great earnestness, that when we fast, we should hallow the fast. For not all those who call upon God, hallow God, since there are some who defile Him; yet not Him--that is impossible--but their own mind concerning Him; for He is holy, and has pleasure in the saints. And therefore the blessed Paul accuses those who dishonour God; “Transgressors of the law dishonour God”. So then, to make a separation from those who pollute the fast, he saith here, “sanctify a fast.” For many, crowding to the fast, pollute themselves in the thoughts of their hearts, sometimes by doing evil against their brethren, sometimes by daring to defraud. And, to mention nothing else, there are many who exalt themselves above their neighbours, thereby causing great mischief. For the boast of fasting did no good to the Pharisee, although he fasted twice in the week, only because he exalted himself against the publican. In the same manner the Word blamed the children of Israel on account of such a fast as this, exhorting them by Isaiah the Prophet, and saying, “This is not the fast and the day that I have chosen, that a man should humble his soul; not even if thou shouldest bow down thy neck like a hook, and shouldest strew sackcloth and ashes under thee; neither thus shall ye call the fast acceptable”. That we may be able to shew what kind of persons we should be when we fast, and of what character the fast should be, listen again to God commanding Moses, and saying, as it is written in Leviticus, “And the Lord spake unto Moses, saying, In the tenth day of this seventh month, there shall be a day of atonement; a convocation, and a holy day shall it be to you; and ye shall humble your souls, and offer whole burnt-offerings unto the Lord.” And afterwards, that the law might be defined  on this point, He proceeds to say; “Every soul that shall not humble itself, shall be cut off from the people”.
 
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 saith; “Being nourished by the word of truth”. Such was the case with our Lord, who said, “My meat is to do the will of My Father which is in heaven”. But if it is not thus with the soul, when 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 Ethiopia”. 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; “Except ye eat My flesh, and drink My blood”. 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 acknowledgement 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.
 
Saint Athanasius: First Paschal Homily (329 AD.)