Saint Isaac the Syrian - On Fasting

The labor of vigil and fasting is the beginning of every struggle against sin and lust..

Fasting is the champion of every virtue, the beginning of the struggle, the crown of the abstinent, the beauty of virginity and sanctity, the resplendence of chastity, the commencement of the path of Christianity, the mother of prayer, the well-spring of sobriety and prudence, the teacher of stillness, and the precursor of all good works.

When the seal of fasting is set upon a man's lips, his thought reflects compunction, his heart pours forth prayer, gloom lies upon his countenance, shameful thoughts are far removed from him, cheer can not be detected in his eyes, and he is an enemy of lust and vain conversations. No one has ever seen a discerning faster enslaved by evil desires...

And the Saviour also, when He manifested Himself to the world in the Jordan, began at this point. For after His baptism the Spirit led Him into the wilderness and He fasted for forty days and forty nights. Likewise all who set out to follow in His footsteps make the beginning of their struggle on this foundation. For this is a weapon forged by God, and who should escape blame if he neglects it? And if the Lawgiver Himself fasts, who among those who keep the law has no need of fasting? This is why the human race knew no victory before fasting,and the devil never experienced defeat from our nature; but this weapon has made him powerless from the outset. Our Lord was the firstborn Leader of victory, so as to set the first crown of victory upon the head of our nature. When the devil, the foe and the tyrant, sees a man bearing this weapon, he is straightway frightened and he recollects and considers the defeat which he suffered in the wilderness at the hands of the Saviour; at once his strength is shattered and the very sight of this weapon, given us by our Commander in chief burns him...

What weapon is more powerful and gives more boldness to the heart in the time of battle against the spirits of wickedness, than hunger endured for Christ's sake?...

Such is the nature of fasting, that whoever perseveres in it will possess an unshakeable mind, one ready to encounter and repel all the troublesome passions.

St. Isaac the Syrian, Ascetical Homilies