The reason that fasting has an effect on the spirits of evil rests in its powerful effect on our own spirit. A body subdued by fasting brings the human spirit freedom, strength, sobriety, purity, and keen discernment.
When you fast and are nourished with abstinence, do not store the leftovers for tomorrow, but, as the Lord became poor and enriched us, feed someone who does not want to be hungry, you who hungers willingly. Then your fast will be like the dove who brings and joyfully proclaims salvation to your soul from the flood.
If a king wanted to take possession of his enemy's city, he would begin by cutting off the water and the food and so his enemies, dying of hunger, would submit to him. It is the same with the passions of the flesh; if a man goes about fasting and hungry the enemies of his soul grow weak.
Woe is he who knowingly chooses to sin with the intention to repent when morning comes, for he knows not what the coming day or the night that precedes it will bring.
Fasting appears gloomy until one steps into its arena. But begin and you will see what light it brings after darkness, what freedom from bonds, what release after a burdensome life…
Fasting was of no benefit to that Pharisee in the Gospel, even though he always fasted two days a week, because he had adulterated it with pride and condemnation of his neighbor (Luke 18:11-12). Not that this means fasting is unprofitable. Moses, Elijah & the Lord Himself showed how beneficial it is for those who fast properly in a way pleasing to God.
A thick rope is composed of thin strands of hemp. One thin strand cannot hold you bound nor strangle you, for you will easily, with the lightest touch, break it and free yourself. But if a thick rope binds you, you will stay bound, and it will strangle you. You cannot easily break it and free yourself of it. As a thick rope is made from thin, weak strands, so men's passions are made up of smaller initial sins. The small, initial sins a man can still break and free himself of. But sin on sin, repeated, the weave becomes thicker and thicker until it becomes a passion, which masters a man as only it can do. You can neither cut it out easily nor cast it away from you nor be divorced from it. Oh, when will men guard themselves from these first sins? Then they would not have so much difficulty in freeing themselves from the passions.
Suppose you have ordered yourself not to eat fish; you will find that the enemy continually makes you long to eat it. You are filled with an uncontrollable desire for the thing that is forbidden. In this way you can see how Adam's fall typifies what happens to all of us. Because he was told not to eat from a particular tree, he felt irresistibly attracted to the one thing that was forbidden him.
God seeks nothing else from us men except that we do not sin; this alone. But this is not a work of law; it is rather a careful guarding of the image and dignity from above. In these things, affirmed in our nature and bearing the radiant garment of the Spirit, we shall abide in God and He in us. We shall be called good, and sons of God by adoption, marked in the light of our knowledge of God.
In this time of fasting and prayer, brethren, let us with all our hearts forgive anything real or imaginary we have against anyone. May we all devote ourselves to love, and let us consider one another as an incentive to love and good works, speaking in defense of one another, having good thoughts and dispositions within us before God and men. In this way our fasting will be laudable and blameless, and our requests to God while we fast will be readily received.
Fasting is the champion of every virtue, the beginning of the struggle, the crown of the abstinent, the beauty of virginity and sanctity, the resplendence of chastity, the commencement of the path of Christianity, the mother of prayer, the well-spring of sobriety and prudence, the teacher of stillness, and the precursor of all good works. Just as the enjoyment of light is coupled with healthy eyes, so desire for prayer accompanies fasting that is practiced with discernment.
There is a sin which is always 'unto death' [1 Jn 5:16]; the sin which we have not repented. Even a saint's prayers will not be heard for the unrepented sin. The person who repents correctly does not imagine that his sins are cancelled through his own effort; but knows that through this effort he makes peace with God.