The reason that fasting has an effect on the spirits of evil rests in its powerful effect on our own spirit. A body subdued by fasting brings the human spirit freedom, strength, sobriety, purity, and keen discernment.
In the evening, on going to sleep (an image of death for the life of that day); examine your actions during the day that has passed. Such an examination is not difficult for one who leads an attentive life, because attention destroys that forgetfulness which is so characteristic of a distracted person. And thus, recalling all your sins in deed, word, thought and feeling, offer repentance over them to God with the disposition and heartfelt promise of correction.
Sear your loins by abstaining from food, and prove your heart by controlling your speech, and you will succeed in bringing the desiring and incensive powers of your soul into the service of what is noble and good.
When you receive from Heaven the gift of patience, be attentive and vigilant over yourself, so as to hold and keep within yourself the grace of God, lest sin should creep unnoticed into your soul or body and drive away this grace.
Having guarded ourselves against distractions and worries, let us turn our attention to our body on which mental vigilance is completely dependent. Human bodies differ widely from one another in strength and health. Some by their strength are like copper and iron; others are frail like grass. For this reason everyone should rule his body with great prudence, after exploring his physical powers. For a strong and healthy body, special fasts and vigils are suitable; they make it lighter, and give the mind a special wakefulness. A weak body should be strengthened by food and sleep according to one's physical needs, but on no account to satiety. Satiety is extremely harmful even for a weak body; it weakens it, and makes it susceptible to disease. Wise temperance of the stomach is a door to all the virtues. Restrain the stomach, and you will enter Paradise. But if you please and pamper your stomach, you will hurl yourself over the precipice of bodily impurity, into the fire of wrath and fury, you will coarsen and darken your mind, and in this way you will ruin your powers of attention and self-control, your sobriety and vigilance…
Confirm yourself in this truth: that every Divine writing that is in agreement with the path of salvation instructs, teaches, chastises, and strengthens, that our path might be ever according to God.
Labor to acquire meekness. Concerning the heavenly virtues, meekness and humility, the Lord Himself teaches us, saying: Learn of Me, for I am meek and humble in heart; and ye shall find rest unto your souls (Matt. 11:29). Learn not from angels, nor from men, but from Me, He says; that is, from the higher wisdom.
Observe your thoughts, and beware of what you have in your heart and your spirit, knowing that the demons put ideas into you so as to corrupt your soul by making it think of that which is not right, in order to turn your spirit from the consideration of your sins and of God.
We believe that the divine presence is everywhere and that 'the eyes of the Lord are looking on the good and the evil in every place.' But we should believe this especially without any doubt when we are assisting at the Work of God. To that end let us be mindful always of the Prophet's words, 'Serve the Lord in fear' and again, 'Sing praises wisely' and 'In the sight of the Angels I will sing praise to Thee.' Let us therefore consider how we ought to conduct ourselves in the sight of the Godhead and of His Angels, and let us take part in the psalmody in such a way that our mind may be in harmony with our voice.
The Holy Fathers teach us how to become familiar with the Gospel, how to read it and how to understand it, what helps and what opposes its understanding. Therefore, at first you must devote more time to reading the Holy Fathers...
He who knows himself pays no heed to the sins of others, but looks at his own and is always repenting over them; he reflects concerning himself, and condemns himself, and does not interfere in anything apart from his own position.