Illusion acts in one way, and God in another -- God the illimitable Master of man, who was and is now their Creator. He who created and re-creates, does He not remain always the Creator? Therefore, beloved brother, listen how the action of illusion differs from divine action. Illusion, when it approaches man in thought or in dream, in some subtle idea, by some apparition which can be seen with the physical eyes, or by a voice from on high heard by the physical ears, never approaches as an absolute master, but comes as a charmer who seeks acceptance by man, and from his acceptance gains power over him. The action of illusion inside or outside man is always action from without; it is open to man to repel it. Illusion is always met at first by a certain doubt in the heart: only those whom it has conquered decisively accept it without question. Illusion never unites a man who is divided by sin, it does not stop the upsurge of blood, does not lead the ascetic to repentance, does not make him small in his own eyes; on the contrary it fires his imagination, encourages the rush of blood, brings him to a certain tasteless, poisonous enjoyment, and flatters him insidiously, inspiring him with self-conceit and establishing in his soul an idol -- 'I'.
Bishop Ignatii Brianchaninov, from The Art of Prayer, p. 147.