Archimandrite Sophrony (Sakharov). "Saint Silouan, the Athonite". Part II. Works of Elder Silouan
If you see a light either within or outside yourself, do not trust it, if you have not got together with this light in your heart adoration to God and love for your neighbour; but have no fear either, just humble yourself, and this light will disappear.
If you see some vision, image, or a dream, give no trust to it, for if it is from Our Lord, then the Lord will give you understanding. The soul that does not know the savour of the Holy Spirit is unable to understand the origin of a vision. The Enemy gives some kind of sweetness mixed with vain self-satisfaction to the soul, and that is how you can understand it is a delusion.
The Fathers say that if a vision is provoked by the Enemy, the soul experiences a certain confusion. But only a humble man, who does not judge himself to be worthy of visions, experiences turmoil or fear under the action of the Enemy; but a vain man may experience neither fear nor even the least distress, for he desires to have visions and judges himself to be worthy of them, and for this reason the Enemy deceives him easily.
What is in heaven is known by the Holy Spirit, what is on earth, by intelligence; whoever tries to know God by the ways of intelligence is in delusion, for God cannot be truly known except by the Holy Spirit.
If you see demons by your spirit, humble yourself and try not to see them, but go without delay to the confessor or to the elder to which you entrusted yourself. Tell everything to your confessor and then the Lord will have compassion on you and you will escape delusion. But if you think that in the domain of the spiritual life you see further than your confessor, if in confession you stop telling him what happens to you, then by that pride you will certainly experience some delusion, by God’s will, in order to give you instruction.