His Holiness Pope Kyrillos the 6th wrote to one of his spiritual sons saying:
Because it is a fact of life that a man will go through different temptations and spiritual wars, God arranged Fasting to deal with temptation and spiritual wars. As God commanded his people in the Old Testament to fast, He also commanded us in the New Testament to fast as the Lord said, “And when he had fasted forty days and forty nights, afterwards He was hungry” (Matt. 4:2) The faithful is a soldier in the spiritual wars, our weapon, we the Christians is prayer and fasting.
As a horse got a bridle, the body also has to overcome its lusts and pleasures. This is what the apostle says, “I say then: Walk in the Spirit, and you shall not fulfill the lust of the flesh. For the flesh lusts against the Spirit, and the Spirit against the flesh; and these are contrary to one another, so that you do not do the things that you wish. But if the Spirit leads you, you are not under the law. Now the works of the flesh are evident, which are: adultery, fornication, uncleanness, lewdness, idolatry, sorcery, hatred, contentions, jealousies, outbursts of wrath, selfish ambitions, dissensions, heresies, envy, murders, drunkenness, revelries, and the like; of which I tell you beforehand, just as I also told you in time past, that those who practice such things will not inherit the kingdom of God. But the fruit of the Spirit is love, joy, peace, longsuffering, kindness, goodness, faithfulness, gentleness, self-control. Against such there is no law. And those who are Christ's have crucified the flesh with its passions and desires. If we live in the Spirit, let us also walk in the Spirit. Let us not become conceited, provoking one another, envying one another.” (Gal. 5:16-24)
When you control your body, the spirit will do its job and the might of the body will be controlled by fasting.
Those who do not fast are called carnal, which means they are lustful. They are exposed always to temptations, and the devil overcomes them. Satan leads them to always follow the desires of their bodies, and keeps them away from eternal life. The devil tempted many, and he led them to lust, they died and fell in sin; among them were our first parents Adam and Eve, Cain, and David. The devil guided Israel until they worshiped foreign Gods. By fasting we overcome the devil and defeat him, and overcome his traps, and conquer his forts; as our Lord said, “However, this kind does not go out except by prayer and fasting.” (Matt. 17:21)
Fasting is a duty to all Christians; our Lord fasted, even though he did not need to fast, but to teach us that the temptations of the devil can not be overcome except by fasting.
By fasting we also get spiritual gifts and we get closer to God, we get our requests, and our demands are answered.
We want fasting to be according to the Spirit where the apostle said, “For those who live according to the flesh set their minds on the things of the flesh, but those who live according to the Spirit, the things of the Spirit. For to be carnally minded is death, but to be spiritually minded is life and peace. Because the carnal mind is enmity against God; for it is not subject to the law of God, nor indeed can be. So then, those who are in the flesh cannot please God. But you are not in the flesh but in the Spirit, if indeed the Spirit of God dwells in you. Now if anyone does not have the Spirit of Christ, he is not His.” (Rom. 8:5-9)
Those who worship God, and seek to live in solidarity with him were not just satisfied with fasts arranged by the church, but they were spending most of their days fasting, like Bishop Daniel the wonder-worker who said, “Because of my long fasting I looked like the dead.” Others fasted all their lives, and never ate good food; as the Apostle Paul said, “Because the carnal mind is enmity against God” (Rom 8:7)
Also one of the bishops was a guest at one of the worshipers in the wilderness, he offered to the bishop bread and salt. After they finished eating, he apologized to the bishop for the kind of food he offered him, so the bishop told him, “Next year I will get neither bread nor salt."
Look my beloved and honored sons at the fathers of our church; how they lived in poverty and piety; and think about the food that the brother offered to the Bishop, and the response of the Bishop to the apology; that he was encouraging that brother in the spirit, not caring about the body.