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).
Beware of the counsels of the evil one, if he should come in the guise of one professing truth to beguile you and lead you into deceit. Even if he should come to you as an angel of light, do not believe him or obey him: for he is apt to fascinate the faithful by the attractive semblance of truth.
The time is coming when people will be insane, and when they see someone who is not insane, they will attack him saying 'You are insane because you are not like us.'
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.
Give thanks to God for all things, because thanksgiving is intercession before God for our weakness. Judge yourself always and in everything as a sinner and as one seduced - and so God will not judge you; be humble in everything and you will receive grace.
If you refuse to accept suffering and dishonor, do not claim to be in a state of repentance because of your other virtues. For self-esteem and insensitivity can serve sin even under the cover of virtue.
But a sober and prudent man who desires to be saved, when he sees from what it is that he suffers harm, carefully preserves himself from evil remembrances, is not drawn into passionate thoughts, avoids meetings and conversations with those for whom he feels attraction and avoids every occasion for sin, fearing lest he himself ignite a fire within himself. This is the warfare which proceeds from one’s own lust, which a man brings on himself
To act 'according to one’s strength' means to use a little less than necessary both of food, and drink, and sleep... As for food, restrain yourself when you wish to eat a little more, and in this way you will always make use of it moderately.
Do not judge and do not belittle anyone, because from this the heart grows faint and the mind is blinded, and from this, negligence appears and unfeelingness of heart is born. Keep ceaseless vigil, learning in the law of God, for through this the heart is warmed by heavenly fire, as is said: 'In my meditation a fire was kindled.'
When you close the doors to your dwelling and are alone you should know that there is present with you the angel whom God has appointed for each man...This angel, who is sleepless and cannot be deceived, is always present with you; he sees all things and is not hindered by darkness. You should know, too, that with him is God, who is in every place; for there is no place and nothing material in which God is not, since He is greater than all things and holds all men in His hand.
When you are praying alone, and your spirit is dejected, and you are wearied and oppressed by your loneliness, remember then, as always, that God the Trinity looks upon you with eyes brighter than the sun; also all the angels, your own Guardian Angel, and all the Saints of God. Truly they do; for they are all one in God, and where God is, there are they also. Where the sun is, thither also are directed all its rays. Try to understand what this means.