Our soul is simple as thought, and rapid as thought or lightning. In an instant it can be wounded by sin and become attached to corruptible things; in an instant it can fall away from the love of God and its neighbor through a single unrighteous thought, through a single passionate desire, through a single malevolent thought, and, therefore, we must constantly watch our heart, lest it should incline to words or thoughts of evil, and must ever strive to preserve it in God's simplicity and purity, and in the love of God and its neighbor.
Those who attend the divine service of the Orthodox Church must bear in mind that its purpose is to prepare us for the joyful service of God in heaven; that in serving God with the body, it is still more necessary to serve God with the soul and a pure heart; that in hearing divine service they must learn to serve God as those saints served him, whose lives, and works of faith, hope and love, we hear of during the service; that God should, above all, be served in deed and truth, and not only by words and the tongue.
Do not be despondent when fighting against the incorporeal enemy, but even in the midst of your afflictions and oppression praise the Lord, Who has found you worthy to suffer for Him, by struggling against the subtlety of the serpent, and to be wounded for Him at every hour; for had you not lived piously, and endeavored to become united to God, the enemy would not have attacked and tormented you.
The Lord commands all men to repent (Matt. 4:17), so that even the spiritual and those making progress should not neglect this injunction and fail to give attention to the smallest and most subtle errors.
To more conveniently become accustomed to remembering God, the fervent Christian has a special means, namely, to repeat ceaselessly a brief prayer of two or three words. Mostly this is 'Lord, have mercy.' or 'Lord Jesus Christ, Son of God, have mercy on me, a sinner.' If you have not yet heard of this, then hear it now, and if you have not done it, then begin from this hour to do it.
The truth and saving properties of our holy faith are clearly revealed by the fact that not a single Sacrament, not a single faithful prayer are left fruitless in us, but they bring and manifest their power from heaven upon our souls and bodies, cleansing our sins, and giving peace to our souls in accordance with the words of our Savior: 'Come unto Me, all ye that labor and are heavy laden, and I will give you rest' (Mt. 11:28); and delivering us from spiritual afflictions and bodily sicknesses! No, we never pray in vain to the Lord, or to the most holy Virgin, or to the angels and saints, but we receive all that we ask them for, and that tends to our salvation.
Most men not only bear Satan's burden willingly in their hearts, but they become so accustomed to it that they often do not feel it, and even imperceptibly increase it. Sometimes, however, the evil enemy increases his burden tenfold, and then they become terribly despondent and fainthearted, they murmur and blaspheme God's name. The usual means that men... take to drive away their anguish are... entertainments... But such means afterwards increase still more the anguish and weariness of their hearts. If, happily, they turn to God, then the burden is removed from their heart, and they clearly see that previously the heaviest burden was lying on their heart, though frequently they did not feel it.
This reversal of thoughts is the strongest means, not only for repelling accidental critical thoughts, but also for completely freeing yourself of this vice…
The enemy of our salvation especially strives to draw our heart and mind away from God when we are about to serve Him, and endeavors to adulterously attach our heart to something irrelevant. Be always, every moment, with God, especially when you pray to Him. If you are inconstant, you will fall away from life, and will cast yourself into sorrow and straitness.
The evil spirit tries to scatter prayer as if it were a sand heap, tries to turn the words into dry sand, without coherency or moisture - that is to say, without fervor of heart. Thus prayer may become either a house built on sand or a house built on a rock. Those build on sand who pray without faith, absently, coldly: such prayer is scattered of itself, and does not bring any profit to him who prays; those build on a rock who, during the whole time of their prayer, have their eyes fixed upon God, and pray to Him as to a living person, conversing face to face with them.
