Excerpt taken from "These Truths We Hold - The Holy Orthodox Church: Her Life and Teachings". Compiled and Edited by A Monk of St. Tikhon's Monastery. Copyright 1986 by the St. Tikhon's Seminary Press, South Canaan, Pennsylvania 18459.
Seeing that bodily disposition is important in worship and spiritual life, in general, great emphasis is placed in the Orthodox Church on fasting; if one should add up all of the fasting seasons and days of the Church calendar, he would find that more than half of the year is devoted to this ascetic labor. The question might rightfully be asked, then, as to why this is so.
According to St. Basil the Great, Adam, the first-created man, loving God of his own free will, dwelt in the heavenly blessedness of communion with God, in the angelic state of prayer and fasting. The cause of this first man's fall was his free will; by an act of disobedience he violated the vow of abstinence and broke the living union of love with God. That is, he held in scorn the heavenly obligations of prayer and fasting by eating of the Tree of Knowledge of Good and Evil. Lack of abstinence, then, was the cause of the Fall and, as a result, because of this original greed, the soul becomes dimmed, and is deprived of the illumination of the Holy Spirit.
Our Lord Jesus Christ calls all of us to salvation through self-denial (Luke 14:26) and this is addressed to the free will of fallen man: If any man would come after Me, let him deny himself and take up his cross and follow Me (Matt. 16:24). Thus, the Savior calls man to the voluntary fulfillment of those heavenly obligations, which he freely forsook, of observing prayer and fasting.
Repentance without fasting is made ineffectual since fasting is the beginning of repentance. The aim of bodily fasting is the enslavement of the flesh, for fasting bridles the lust of the stomach and of that below the stomach, meaning the removal of the passions, the mortification of the body and the destruction of the sting of lust. Thus it is necessary to overcome the stomach for the healing of the passions.
The personal example of the Lord Himself bears witness to the absolute necessity of bodily fasting. Did not the Savior fast for forty days and nights after His baptism to prepare for His earthly ministry (Matt. 4:2)? So too, many of the Saints of the Church were especially noted for their ascetic labors, which saw fasting as being of especially great importance.
In fasting the flesh and the spirit struggle one against the other. Therefore bodily fasting leads to the triumph of the spirit over the body, and gives a man power over the stomach, subdues the flesh and permits it not to commit fornication and uncleanness. Abstinence is the mother of cleanliness, the giver of health and is good for rich and poor, sick and healthy, alike. It strengthens the seeker after godliness in spiritual battles and proves to be a formidable weapon against evil spirits. As the Lord Himself said, concerning the casting-out of certain demons: This kind never cornea out except by prayer and fasting (Matt. 17:21).
This fasting, however, is not to be done out of pride or self-will; It must be observed in the praise of God and must be in accordance with the canons of the Church, since it consists in the complete renunciation of self-will and of the desires. At the same time, we must realize that for fallen man to attain perfection, even intensive fasting is insufficient, if in his soul he does not abstain from those things which further sin. Fasting is not only the abstinence from food, but also from evil thoughts and all passion, for, as the Savior says: Do you not see that whatever goes into the mouth passes into the stomach, and so passes on? But what conies out of the mouth proceeds from the heart, and this defiles a man. For out of the heart come evil thoughts, murder, adultery, fornication, theft, false witness, slander. These are what defile a man... (Matt. 15:17-20). Thus exterior fasting, without the corresponding interior fasting is in vain.