For this healer of our souls is effective, in the case of one to quiet 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, in yet another to restore purity of mind and to set him free from evil thoughts. In one it will control his unbridled tongue, 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 a fountain of compunction to spring forth...we shall readily, with God's help, cleave through the whole sea of passions and pass through the waves of temptations inflicted by the cruel tyrant, and so come to anchor in the port of impassibility.
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.
--St. Symeon the New Theologian