Let each one of us keep in mind the benefit of fasting… For this healer of our souls is effective, in the case of one to quieten the fevers and impulses of the flesh, in another to assuage bad temper, in yet another to drive away sleep, in another to stir up zeal, and in yet another to restore purity of mind and to set him free from evil thoughts. In one it will control his unbridled tongue and, as it were by a bit, restrain it by the fear of God and prevent it from uttering idle and corrupt words. In another it will invisibly guard his eyes and fix them on high instead of allowing them to roam hither and thither, and thus cause him to look on himself and teach him to be mindful of his own faults and shortcomings.
Fasting gradually disperses and drives away spiritual darkness and the veil of sin that lies on the soul, just as the sun dispels the mist. Fasting enables us spiritually to see that spiritual air in which Christ, the Sun who knows no setting, does not rise, but shines without ceasing. Fasting, aided by vigil, penetrates and softens hardness of heart. where once were the vapors of drunkenness it causes fountains of compunction to spring forth.
I beseech you, brethren, let each of us strive that this may happen in us! Once this happens we shall readily, with God’s help, cleave through the whole sea of passions and pass through the waves of the temptations inflicted by the cruel tyrant, and so come to anchor in the port of impassibility.
My brethren, it is not possible for these things to come about in one day or one week! They will take much time, labor, and pain, in accordance with each man’s attitude and willingness, according to the measure of faith and one’s contempt for the objects of sight and thought. In addition, it is also in accordance with the fervor of his ceaseless penitence and its constant working in the secret chamber of his heart that this is accomplished more quickly or more slowly by the gift and grace of God. But without fasting no one was ever able to achieve any of these virtues or any others, for fasting is the beginning and foundation of every spiritual activity.
— Symeon the New Theologian: the Discourses, pub. Paulist Press. pp. 168-169.