Fasting is an ordinance of the Church, obliging the Christian to observe it on specific days. Concerning fasting, our Savior teaches: “When thou fastest, anoint thine head, and wash thy face; that thou appear not unto men to fast, but unto thy Father Who is in secret: and thy Father, Who seeth in secret, shall reward thee openly” (Matt. 6:17-18). From what the Savior teaches we learn (a) that fasting is pleasing to God, and (b) that he who fasts for the uplifting of his mind and heart towards God shall be rewarded by God, Who is a most liberal bestower of Divine gifts, for his devotion.
Fasting was observed by the holy Apostles. In the Acts of the Apostles we read: “As they ministered to the Lord, and fasted, the Holy Spirit said…” (Acts 13:2). And immediately after this, we read: “When they had fasted and prayed, and laid their hands on them, they sent them away” (Acts 13:3). And in another chapter we read: “And when they had ordained for them elders in every church, and had prayed with fasting, they commended them to the Lord” (Acts 14:23).
In the New Testament fasting is recommended as a means of preparing the mind and the heart for divine worship, for long prayer, for rising from the earthly, and for spiritualization.
Fasting is enjoined in the Old Testament also, and was practiced by the Jews.
The purpose of fasting is chiefly spiritual: to provide an opportunity and preparation for the spiritual works of prayer and meditation on the Divine through the complete abstinence from food, or the eating of uncooked food or frugal fare. However, fasting is no less valuable for physical health, since self-control and simplicity of life are necessary conditions of health and longevity, as dietetics tells us.
-From Modern Orthodox Saints: St. Nectarios of Aegina by Constantine Cavarnos, Belmont, MA: Institute for Byzantine and Modern Greek Studies, 1981.