Saint Peter of Damaskos. "Book 1. A Treasury of Divine Knowledge." From Philokalia, Vol. 3.
In addition, when the devil saw Christ descending in His extreme goodness to the holy martyrs and revered fathers, appearing either in Himself or through angels or in some other ineffable form, he began to fabricate numerous delusions in order to destroy people. It is on account of this that the fathers, in their discrimination, wrote that one should not pay any attention to such diabolic manifestations, whether they come through images, or light, or fire, or some other deceptive form.’ For the devil can deceive even in sleep or through the senses. If we accept such delusions, he makes the intellect, in its utter ignorance and self-conceit, depict various shapes or colors so that we think that this is a manifestation of God or of an angel. Often in sleep, or to our senses when awake, he shows us demons that are apparently defeated. In short, he does all he can to destroy us by making us succumb to these delusions.
In spite of all this, the devil will fail in his purpose if we apply the counsel of the holy fathers: that during the time of prayer we should keep our intellect free from form, shape, and colour, and not give access to anything at all, whether light, fire or anything else; and that we should do all we can to confine our mind solely to the words we are saying, since he who prays only with his mouth prays to the wind and not to God. For, unlike men, God is attentive to the intellect and not to the words spoken. We must worship, it is said, ‘in spirit and in truth’ (John 4:24); and again, ‘I had rather speak five words whose meaning I understand than ten thousand words in a strange tongue’ (1 Cor. 14:19).