St. Ignatius Brianchaninov (d. 1867) writes:
“If you have felt that your mind has come to be at one with the soul and body, that you are no longer cut into pieces by sin but are something unified and whole, that the hallowed peace of Christ is breathing in you, then watch over this gift of God with all possible care. Let prayer and the reading of religious books be your principal occupation; give to other works only a secondary importance, be cold towards earthly activities, and if possible eschew them altogether. Sacred peace, fine as the breath of the Holy Spirit, immediately withdraws from the soul which behaves carelessly in its presence; the soul which lacks reverence, proves disloyal by indulging in sin, and permits itself to grow negligent. Together with the peace of Christ, grace-given prayer withdraws likewise from the unworthy soul: then the passions invade it like hungry beasts, and begin to torment the victim who has given himself to them, and who has been left to himself by God, who has withdrawn from him.
If you become surfeited with food, or still more with drink, the peace of God will cease to act in you.
If you are angry, you will lose this peace for a long while.
If you allow yourself to become irreverent, the Holy Spirit will no longer work within you.
If you begin to love something earthly, if you become infected by a passionate attachment to some object or skill, or by a special liking for some person, holy peace will certainly withdraw from you.
If you allow yourself to take pleasure in impure thoughts, peace will leave you for a long time, because it does not tolerate the evil stench of sin – and especially the sins of lust and vanity.
You will seek this peace and find it not; you will weep for its loss; but it will pay no attention to your tears, that so you may learn to give due value to the divine gift, and to guard it with proper care and reverence. Hate everything that draws your down into distraction or sin. Crucify yourself on the cross of the Gospel commandments; keep yourself always nailed to it. Rebuff all sinful thought and wishes with courage and vigilance; cast away earthly care; try to live the Gospel by zealously fulfilling all its commandments. When you pray, once more crucify yourself on the cross of prayer. Push aside all the memories, however important they may be, which come to your during prayer: ignore every one of them. Do not theologize; do not be carried away by following up brilliant, original, and powerful ideas which suddenly occur to you. Sacred silence, which is induced in the mind at the time of prayer by a sense of God’s greatness, speaks of God more profoundly and more eloquently than any human word. ‘If you pray truly,’ said the Fathers, ‘you are a theologian.’” (The Art of Prayer: An Orthodox Anthology compiled by Igumen Chariton of Valamo, pp 207-208)