The late Athonite Father Tikhon used to say: The prayer, 'Lord Jesus have mercy on us' is worth one hundred drachmas, but 'Glory to God' is word one thousand. Glorifying God is more valuable than anything else, because in the first instance, people often say the Jesus Prayer when needing something; but when one glorifies God in the midst of suffering, it is an ascesis.
Therefore, if you wish to conquer the passions, cut off the love of pleasure; but if you are pursuing food, you will spend a life in passions; the soul will not be humbled if the flesh is not deprived of bread. It is not possible to deliver the soul from perdition while protecting the body from unpleasantness.
Peter was first given the keys, but then he was allowed to fall into the sin of denying Christ; and so his pride was humbled by his fall. Do not be surprised, then, if after receiving the keys of spiritual knowledge you fall into various evil thoughts. Glorify our Lord, for He alone is wise: through setbacks of this kind He restrains the presumption that we tend to feel because of our advance in the knowledge of God. Trials and temptations are the reins whereby God in His providence restrains our human arrogance.
Acquire humility, which scorches the demons, obedience, which opens the door for the Son of God to enter a man, faith which saves a man, hope which makes him unashamed; and love which lets not a man fall away from God.
What should one do so that the mind might be constantly occupied with God? If we do not acquire the three following virtues: love for God and men, continence, and the Prayer of Jesus, then our mind cannot be completely occupied with God. For love makes anger meek, continence weakens fleshly desire, and prayer draws the mind away from thoughts and banishes every hatred and high-mindedness.
One time St. Nicodemos, on a feast day, was walking toward the Great Lavra (on Mt Athos). On his way he came across a kellion where he spent the night. At midnight he saw an elder and his accompanying monks entering the church. He secretly went in also, and there he saw the elder and his subordinate monks uttering the Jesus Prayer ('Lord Jesus Christ, Son of God, have mercy on me'), sometimes in a kneeling position, sometimes standing up. And at the time of the Holy Communion, he saw all their faces shining only a little less dimly than the sun.
Concerning the fault of our brother, seeing and hearing of it, be toward it as a blind man, deaf man, and dumb one, not seeing, not hearing, and not speaking, as one crude of mind, not understanding, not showing oneself to be wise. But to oneself be attentive, discerning, and clear-sighted.
Even a pious person is not immune to spiritual sickness if he does not have a wise guide -- either a living person or a spiritual writer. This sickness is called prelest, or spiritual delusion, imagining oneself to be near to God and to the realm of the divine and supernatural. Even zealous ascetics in monasteries are sometimes subject to this delusion, but of course, laymen who are zealous in external struggles (podvigi) undergo it much more frequently. Surpassing their acquaintances in struggles of prayer and fasting, they imagine that they are seers of divine visions, or at least of dreams inspired by grace. In every event of their lives, they see special intentional directions from God or their guardian angel. And then they start imagining that they are God's elect, and often try to foretell the future. The Holy Fathers armed themselves against nothing so fiercely as against this sickness -- prelest.
Concerning fasting, do not grieve, as I have said to you before: God does not demand of anyone labors beyond his strength. And indeed, what is fasting if not a punishment of the body in order to humble a healthy body and make it infirm for passions, according to the word of the Apostle: 'When I am weak, then am I strong' (II Corinthians 12:10).
If thou, being offended by anything, dost sense that grief and wrath have seized thee, preserve silence, and say naught until unceasing prayer pacifies thine heart.