Saint Gregory of Sinai. "On Commandments and Doctrines, Warnings and Promises; On Thoughts, Passions and Virtues, and Also on Stillness and Prayer: One Hundred and Thirty-Seven Texts". From Philokalia, Vol. 4.
131. Here something must be said about delusion, so far as this is possible; for, because of its deviousness and the number of ways in which it can ensnare us, few recognize it clearly and for most it is almost inscrutable. Delusion manifests itself or, rather, attacks and invades us in two ways - in the form of mental images and fantasies or in the form of diabolic influence - though its sole cause and origin is always arrogance. The first form is the origin of the second and the second is the origin of a third form - mental derangement. The first form, illusory visions, is caused by self-conceit; for this leads us to invest the divine with some illusory shape, thus deceiving us through mental images and fantasies. This deception in its turn produces blasphemy as well as the fear induced by monstrous apparitions, occurring both when awake and when asleep - a state described as the terror and perturbation of the soul. Thus arrogance is followed by delusion, delusion by blasphemy, blasphemy by fear, fear by terror, and terror by a derangement of the natural state of the mind. This is the first form of delusion, that induced by mental images and fantasies.
The second form, induced by diabolic influence, is as follows. It has its origin in self-indulgence, which in its turn results from so-called natural desire. Self-indulgence begets licentiousness in all its forms of indescribable impurity. By inflaming man's whole nature and clouding his intelligence as a result of its intercourse with spurious images, licentiousness deranges the intellect, searing it into a state of delirium and impelling its victim to utter false prophecies, interpreting the visions and discourses of certain supposed saints, which he claims arc revealed to him when he is intoxicated and befuddled with passion, his whole character perverted and corrupted by demons. Those ignorant of spiritual matters, beguiled by delusion, call such men 'little souls'. These 'little souls' are to be found sitting near the shrines of saints, by whose spirit they claim to be inspired and tested, and whose purported message they proclaim to others. But in truth they should be called possessed by the demons, deceived and enslaved by delusion, and not prophets foretelling what is to happen now and in the future. For the demon of licentiousness himself darkens and deranges their minds, inflaming them with the fire of spiritual lust, conjuring up before them the illusory appearance of saints, and making them hear conversations and see visions. Sometimes the demons themselves appear to them and convulse them with fear. For having harnessed them to the yoke of Belial, the demon of licentiousness drives them on to practice their deceits, so that he may keep them captive and enslaved until death, when he will consign them to hell.
132. Delusion arises in us from three principal sources: arrogance, the envy of demons, and the divine will that allows us to be tried and corrected. Arrogance arises from superficiality, demonic envy is provoked by our spiritual progress, and the need for correction is the consequence of our sinful way of life. The delusion arising solely from envy and self-conceit is swiftly healed, especially when we humble ourselves. On the other hand, the delusion allowed by God for our correction, when we are handed over to Satan because of our sinfulness, God often permits to continue until our death, if this is needed to efface our sins. Sometimes God hands over even the guiltless to the torment of demons for the sake of their salvation. One should also know that the demon of self-conceit himself prophesies in those who are not scrupulously attentive to their hearts.