Three old men once came to Abba Sisoes [St. Sisoes the Great] because they had heard that he was a great man. And the first one said unto him, "Father, how can I escape from the river of fire?" And Abba Sisoes answered him never a word. Then the second old man said to him, "Father, how can I escape from the gnashing of teeth, and from the worm which never dies?" And Abba Sisoes answered him never a word. Then the third old man said to him, "Father, what will I do? For the remembrance of the outer darkness troubles me." And Abba Sisoes answered and said to them, "I never think on any of these things, but I believe that God is Merciful, and that He will show mercy to me"; then the old men went away grieved at the answer which Abba Sisoes had spoken to them. Now because he did not wish to send them away sorrowful, he brought them back, and said to them, "Blessed are ye, O my brothers, for I have been jealous of you"; and they said to him, "In what matter have you been jealous of us?" And he said, "The first one of you spake about a river of fire; and the second spake about the gnashing of teeth and the worm which dies not; and the third spake about the outer darkness; if remembrances of this kind have dominion over your minds it is impossible for you to commit sin. What can I do who am stubborn of heart? For hardness of heart will not allow me to perceive even that there a punishment for men existeth, and because of this I sin every hour." And when the old men had heard these words, they made excuses to him, and said, "In very truth according to what we have heard, even so have we seen."

from Sayings of the Holy Desert Fathers

Be concentrated without self-display, withdrawn into your heart. For the demons fear concentration as thieves fear dogs.

St. John Climacus

When you begin to read or listen to the Holy Scriptures, pray to God thus: “Lord Jesus Christ, open the ears and eyes of my heart so that I may hear Your words and understand them, and may fulfill Your will.” Always pray to God like this, that He might illumine your mind and open to you the power of His words. Many, having trusted in their own reason, have turned away into deception.

St. Ephraim the Syrian

In order that you may move your will more easily to this one desire, in everything—to please God and to work for His glory alone—remind yourself often that He has granted you many favors in the past and has shown you His love. He has created you out of nothing in His own likeness and image, and has made all other creatures your servants; He has delivered you from your slavery to the devil, sending down, not one of the angels, but His Only-begotten Son to redeem you, not at the price of corruptible gold and silver, but by His priceless blood and His most painful and degrading death. Having done all this He protects you, every hour and every moment, from your enemies; He fights your battles by His divine grace; in His immaculate Mysteries He prepares the Body and Blood of His beloved Son for your food and protection. All this is a sign of God’s great favor and love for you; a favor so great that it is inconceivable how the great Lord of hosts could grant such favors to our nothingness and worthlessness.

St. Nicodemus of life Holy Mountain

...let us hold fast to love of Him, hating and rightly turning away from the devil. For as our Benefactor is loved and cherished in proportion to His benefactions, so the wicked one should be hated and rejected for his ways in equal proportion. For he is the destroyer of our life. In the words of the Master, he is a murderer from the beginning [John 8:44]. He is the one who has divided our race into ten thousand opinions, wounding it with many darts of sin and seeking to swallow down the inhabited world. If we do not hate him, there will be no escaping the punishment that will be meted out to us, because we joined to our foe and murderer. But, my brothers, let us fly from him! Let us fly most certainly. What is flight? The avoidance of wicked actions and thoughts, and also affinity with God, the assumption of good works.

St. Theodore the Studite

Grumbling is caused by misery and it can be put aside by doxology (giving praise). Grumbling begets grumbling and doxology begets doxology. when someone doesn’t grumble over a problem troubling him, but rather praises God, then the devil gets frustrated and goes off to someone else who grumbles, in order to cause everything to go even worse for him. You see, the more one grumbles, the more one falls into ruin. 

Sometimes the devil deceives us and makes us unable to be pleased with anything; however, one can celebrate all things in a spiritual manner, with doxology, and secure God’s constant blessing.

St. Paisios of Mount Athos

Christ prayed for those that crucified Him: ‘Father, count not this sin against them; they know not what they do.’ Archdeacon Stephen prayed for those who stoned him so that the Lord would not judge this sin against them. And so we, if we wish to retain grace, must pray for our enemies. If you do not find pity on a sinner who will suffer in flames, then you do not carry the grace of the Holy Spirit, but rather an evil spirit; and while you yet live, you must free yourself from his clutches through repentance.

St. Silouan the Athonite

We must consider all evil things, even the passions which war against us, to be not our own, but of our enemy the devil. This is very important. You can only conquer a passion when you do not consider it as part of you.

St. Nikon of Optina

When the soul of a man departs from the body, a certain great mystery is there enacted. If a person is under the guilt of sin, bands of demons and fallen angels approach along with the powers of darkness which capture the soul and drag it as a captive to their place. No one should be surprised by this fact. For if, while a man lived in this life, he was subject to them and was their obedient slave, how much more, when he leaves this world, is he captured and controlled by them?

St. Macarius the Great

Why do demons wish to excite in us gluttony, fornication, greed, anger, rancor and other passions? So that the mind, under their weight, should be unable to pray as it ought; for when the passions of our irrational part begin to act, they prevent the mind from acting rationally.

St. Nilus of Sinai

While the dying person addresses his last words to us, suddenly his tongue is at a loss, his eyes dim, his mouth falls silent, his voice paralyzed when the Lord’s troops have arrived, when His frightening armies overwhelm him, when the divine bailiffs invite the soul to be gone from the body, when the inexorable lays hold of us to drag us to the tribunal… Then the angels take the soul and go off through the air. There stand principalities, powers and leaders of the adverse troops who govern the world, merciless accusers, strict agents of an implacable tax bureau, like so many examiners that await the soul in the air, ready to demand a reckoning, to examine everything, brandishing their claims, that is to say our sins: those of youth and of old age, those intentional and those not so, those committed by actions and those by words or thoughts. Great then is the fear of the poor soul, inexpressible its anguish when it sees itself at grips with these myriads of enemies, who stop it, push and shove it, accuse it, hinder it from dwelling in the light, from entering into the land of the living. But the holy angels, taking the soul, lead it away.

St. Ephraim of Syria