Saint Symeon Metaphrastis. "Paraphrase of the Homilies of Saint Makarios of Egypt". III Patient Endurance and Discrimination. From Philokalia, Vol. 3.
45. He who follows the spiritual path must pay great attention to discrimination, since the ability to distinguish between good and evil, and to scrutinize and understand the various tricks through which the devil by means of plausible fantasies leads most people astray, keeps us safe and helps us in every way. If a man wanting to test his wife’s virtue comes to her at night disguised as someone else, and she repels him, he will rejoice at this and welcome the assurance it gives. It is exactly the same with us in relation to the attacks of the evil spirits. Even if you repel the heavenly spirits, they will be gladdened by this, and will help you to participate still further in grace: because of this proof of your love for the Lord they will fill you brim-full with spiritual delight. So do not from light-mindedness speedily surrender yourself to the visitations of spirits, even if they are heavenly angels, but be wary, submitting them to the most careful scrutiny. Thus you will welcome the good and repel the evil. In this way you will increase in yourself the workings of grace, which sin, however much it may assume the appearance of the good, cannot altogether simulate. According to St Paul, Satan can even change himself into an angel of light in order to practice his deceptions (cf. 2 Cor. 11:14); yet though he may manifest himself in such a glorious manner, he cannot, as we said, produce within us the effects of grace, and so it becomes quite clear that the vision is counterfeit. For the devil cannot bring about love either for God or for one’s neighbor, or gentleness, or humility, or joy, or peace, or equilibrium in one’s thoughts, or hatred of the world, or spiritual repose, or desire for celestial things; nor can he quell passions and sensual pleasure. These things are clearly the workings of grace. For the fruits of the Spirit are love, joy, peace, and so on (cf. Gal. 5:22), while the devil is most apt and powerful in promoting vanity and haughtiness. You may know from its effect whether the intellectual light shining in your soul is from God or from Satan. Indeed, once it has developed its powers of discrimination, the distinction is immediately clear to the soul itself through intellectual perception. Just as the throat through its sense of taste distinguishes the difference between vinegar and wine, although they look alike, so the soul through its intellectual sense and energy can distinguish the gifts of the Spirit from the fantasies of the devil.