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That is why you must above all lovingly study the holy scriptures, why your soul must be illuminated by divine utterances, why the dark shadows of the devil have to be dispersed by the flash of God’s word; for the devil is quick to flee from the soul which is illuminated by divine speech, which is always occupied with heavenly thoughts, in which God’s word, whose force the evil spirit is unable to endure, is constantly present. For that reason the blessed apostle compared it to a sword when listing the arms for use in the war of the spirit (Eph.6.17). It is perfectly safe, however, for the mind to become accustomed to differentiating between one thought and another - always subject, of course, to careful and watchful control - and, at the first stirring of the mind, either to approving or to disapproving of what it is thinking, so that it either nourishes good thoughts or immediately destroys bad ones. In this lie the source of good and the origin of sin, and thought is the beginning of every great offense in the heart, painting every single deed on the tablet of the heart, as it were, before doing it; for every deed and every word, whichever it may be, is laid out for inspection in advance and its future is decided by thoughtful consideration. You can see what a brief moment sometimes separates a man’s thinking each thought and his putting that thought into action, nor is anything at all done by the tongue or the hand or other limbs, unless thoughts have previously dictated it; hence the Lord also says in the gospel: Out of the heart of man come evil thoughts, adultery, fornication, murder, theft, false witness, greed, evil, trickery, unchastity, the evil eye, blasphemy, pride, folly. These are what defile men (Mt.15.19,20)
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