St Isaac the Syrian was born in Nineveh in the 6th century. He entered the monastery as a youth and having perfected himself and succeeding in good deeds, he isolated himself in a desert cave to maintain silence, heeding only to himself and God. After a number of years, God summoned him to serve as the ruling bishop of the Nineveh Church.
However, after being ordained, Saint Isaac did not remain very long as bishop. This occurred because of the disobedience of a moneylender, who refused to act according to the Gospel. The Saint thought: "If they are not obeying God’s directives in the Gospel, then what remains for me to do here?" He returned to his beloved hermitage in the desert. Here he lived in silence up to his death, abiding in deeds of self-denial, in struggles with the flesh and temptations of the demons. Saint Isaac wrote much, relying on his vast spiritual experiences. His instructions came to us in the form of 91 homilies. He reposed in the 6th century. The Church commemorates St Isaac on 28 January.
When a man begins to fast, he straightway yearns in his mind to enter into converse with God. For the body that fasts cannot endure to sleep upon its pallet all the night through. Fasting naturally incites wakefulness unto God, not only during the day, but also at night. For the empty body of a faster is not greatly wearied by the battle against sleep. And even if his senses are weakened, his mind is wakeful unto God in prayer. It is better for a man to desist from his liturgy because of weakness due to fasting, than because of sloth due to eating. When the seal of fasting is set upon a man's lips, his thought reflects with compunction, his heart pours forth prayer, gloom lies upon his countenance, shameful thoughts are far removed from him, cheer cannot be detected in his eyes, and he is an enemy of lusts and vain conversations.
No one has ever seen a discerning faster enslaved by evil desires. Fasting with discernment is a spacious mansion for every good thing; but he who neglects fasting makes every good totter. For fasting, was the commandment that was given to our nature in the beginning [to Adam and Eve] to protect it with respect to the tasting of food, and in this point the progenitor of our substance fell. There, however, where the first defeat was suffered, the ascetic strugglers make their beginning in the fear of God as they start to keep His laws. 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 upon this foundation.
For this is a weapon forged by God, and who shall 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 had never experienced defeat from our nature; but this weapon has made him powerless from the outset.
Once, a demonized youth went to the skete of some Fathers in order to be cured by their prayers. Out of humility, however, they fled. For much time, the unfortunate man suffered thus, until a certain elder took pity on him, crossed him with a wooden cross he had on his belt, and cast out the evil spirit. "Since you evict me from my abode," the spirit said, "I will enter you. "Come," the elder courageously answered him. So it is that the demon entered into him and tortured him for a full twelve years. The holy one endured the struggle with fortitude, but fought against his enemy with superhuman fasting and unceasing prayer. All of those years he never even once put food in his mouth, chewing rather a few date pits every evening and swallowing only the juice from them. Finally, conquered by the incessant struggle of the elder, the demon freed him. "Why are you leaving?" the elder asked him. "No one is casting you out." "Your fasting destroyed me," answered the demon, becoming invisible.
From The Ascetical Homilies of St Isaac The Syrian