Fasting, by definition, means abstaining from food, drink, and pleasures. Sometimes abstinence is manifested by not eating anything at all (for one or several days). Most frequently, however, it is accomplished by giving up meat and dairy products and switching to vegetarian foods. In medical terms fasting is similar to dieting. It is a diet with an important spiritual goal.
The necessity of abstinence of food arises from the human constitution itself, possessing not only a body but an immortal soul. Because of the sinfulness of our nature, the harmony between our body and soul has been disturbed. As a result, the desires of the flesh often predominate and sometimes completely stifle all the soul's endeavors for righteousness. A person becomes a pitiable slave to his passions and at times worse than an animal. It is possible to restrain one's physical desires and to allow the soul's noble aspirations to bloom and strengthen with the help of prayer and fasting.
It should be noted that the commandment of abstinence was given to our forefathers Adam and Eve when they were still in paradise and, hence, were sinless. We refer here to the commandment forbidding them to eat the fruits from the tree of the knowledge of good and evil (Genesis 2:17). Fasting became especially necessary after their fall, and we see in the Bible that righteous people throughout the Old and New Testaments fasted during certain periods of their lives. A few examples follow.
The great prophet Moses, the law-giver of the Old Testament, had been fasting for 40 days before he received the Ten Commandments from God on Mt. Sinai (Exodus 34:38). The righteous king David fasted frequently as we can conclude from his God-inspired psalms. The great prophet Elijah, (1 Kings ch.19) who was taken up to heaven alive, also used to fast. The prophet Daniel had fasted before he received the revelation from God about the destiny of his people (Acts 10). The prophet John the Baptist fasted to a great extent and also taught his disciples to do so. The prophetess Anna, living at the temple for about fifty years, served God by fasting and praying throughout the day and night. For that she became worthy of God's grace and received His revelation about the birth of Jesus Christ (Luke 2:37).
Even the sinless God-and-man our Lord Jesus Christ fasted for 40 days to prepare for His mission of the salvation of the world. Following His example, the Apostles and the early Christians used to fast also, as can be found in the epistles of Saint Paul (Acts 13:3; 1 Cor. 7:5 and 9:27; 2 Cor. 6:5 and 11:27). From early Church history we learn that the dedication of particular days of the year to fasting became a widespread practice among Christians in the first few centuries. That is why in our time also the Church gives such great importance to Lent and states that without prayer and fasting spiritual growth is impossible.
The books of the New Testament teach about the benefits of fasting. In answer to the Pharisees' reproach that Christ's disciples did not fast, the Lord answered that the time to fast had not yet arrived because the Bridegroom (Christ) was with them. But when the Bridegroom will be taken away (that is, when Christ dies), then they will fast (Luke 5:33-35). Therefore, since apostolic times it has become customary to fast on Wednesdays, when Judas betrayed Christ, and on Fridays, when Jesus Christ was crucified on the cross. For the same reason the Church timed Lent to the days preceding the Passion Week. To the disciples' question as to why they could not drive out a demon, the Lord answered: "This kind does not go out except by prayer and fasting" (Matthew 17:21). In fact, demons for the most part work through our carnal nature, arousing in it improper passions, thus pushing us towards all kinds of sins, and in this way controlling our will.
To free ourselves of their influence, it is necessary to weaken the body and strengthen the soul through abstinence and prayer. Of course, one has to fast for the sake of improving oneself and not to be praised by people, as the Lord explained in his Sermon on the Mount. He said: "So that you do not appear to men to be fasting, but to your Father who is in the secret place; and your Father who sees in secret will reward you openly" (Matthew 6:18).
The heirs of the Apostles, the Holy Fathers and teachers of the Church, impressed on Christians the importance of fasting during Lent. "Do not scorn the Forty day Fast," writes Saint Ignatius of Antioch to the Philippians. "It is the imitation of the life of Christ." The Blessed Jerome in the name of all Christians of his time said, "We fast during Lent according to the tradition of the Apostles." "The longer the abstinence, the easier the acquirement of salvation," teaches Blessed Augustine. According to the teachings of St. Asterius of Amasis, Lent is "the teacher of moderation, the mother of virtue, the tutor of God's children, the instructor of the confused, the tranquility of thoughts, the support of life, a lasting and undisturbed peace; its strictness and importance weaken passion, extinguish anger and rage, quench and calm any worries which arise from overeating."
Blessed John Kolov said: "When a king plans to capture an enemy's city, he first of all stops its supply of provisions. Then its citizens, pressed by hunger, submit to him. Something similar happens with carnal desires: if a person will spend his life in fasting and hunger then improper desires will fade away." According to the teaching of John Chrysostom, "Just as non-restraint from food is, at times, the cause of countless evils for humanity, so fasting and contempt for carnal pleasures were always the cause of great blessings … As light sailing vessels speedily cross the seas and those overburdened with cargo sink, so fasting, clearing up our mind, helps us to cross the turmoils of our present life and to strive for heaven and spiritual things."
Fasting, according to the teaching of Basil the Great, brings forth prophets, strengthens the warriors of Christ, and makes the law-givers wiser. Fasting is the good guardian of the soul, the weapon of the valiant. It repels temptations, is the cohabitant of sobriety and the foundation of chastity. Fasting carries prayer to heaven, becoming its wings.
The Holy Fathers, explaining the importance of abstaining from food, insisted that one should abstain simultaneously from vices because the moral improvement of a Christian is the main goal of fasting.
"The benefit of fasting," teaches Saint Basil the Great, "is not limited by the abstinence of food alone, because true fasting is the eradication of evil deeds. Everyone should become liberated from lies. Forgive your neighbor when he insults you; forgive him his debts. You don't eat meat, but hurt your brother … We will fast in a way that is pleasing to God. A true fast is the elimination of evil, restraint of what one says, suppression of anger, the alienation of lust, malignant gossip, lies, and perjury. Abstinence from all these is true fasting." In summary, just as we fell into sin and lost heavenly bliss because of the lack of restraint of our forefathers, so through voluntary fasting we can restore in ourselves the grace of God.