Evagrios the Solitary. “On Prayer: One Hundred and Fifty-Three Texts”.
(In Russian Philokalia, this text is attributed to St Nilus of Sinai).
94. Take care that the crafty demons do not deceive you with some vision; be on your guard, turn to prayer and ask God to show you if the intellection comes from Him and, if it does not, to dispel the illusion at once. Do not be afraid, for if you pray fervently to God, the demons will retreat, lashed by His unseen power.
95. You should be aware of this trick: at times the demons split into two groups; and when you call for help against one group, the other will come in the guise of angels and drive away the first, so that you are deceived into believing that they are truly angels.
114. Never try to see a form or shape during prayer.
115. Do not long to have a sensory image of angels or powers or Christ, for this would be madness:it would be to take a wolf as your shepherd and to worship your enemies, the demons.
116. Self-esteem is the start of illusions in the intellect. Under its impulse, the intellect attempts to enclose the Deity in shapes and forms.
134. There are times when the demons suggest thoughts to you and then urge you to rebut them with prayer; whereupon they withdraw of their own accord, so as to deceive you into imagining that you have begun to overcome such thoughts and to rout the demons.