St Gregory of Sinai. "On Stillness: Fifteen Texts. Different Ways of Psalmodizing." From Philokalia, Vol. 4.
10. So, lover of God, attend with care and intelligence. If while engaged in spiritual work you see a light or a fire outside you, or a form supposedly of Christ or of an angel or of someone else, reject it lest you suffer harm. And do not pay court to images, lest you allow them to stamp themselves on your intellect. For all these things that externally and inopportunely assume various guises do so in order to delude your soul. The true beginning of prayer is the warmth of heart that scorifies the passions, fills the soul with joy and delight, and establishes the heart in unwavering love and unhesitating surety. The holy fathers teach that if the heart is in doubt about whether to accept something either sensory or conceptual that enters the soul, then that thing is not from God but has been sent by the devil. Moreover, if you become aware that your intellect is being enticed by some invisible power either from the outside or from above, do not trust in that power or let your intellect be so enticed, but immediately force it to continue its work. Unceasingly cry out: ‘Lord Jesus Christ, Son of God, have mercy’, and do not allow yourself to retain any concept, object, thought or form that is supposedly divine, or any sequence of argument or any color, but concentrate solely on the pure, simple, formless remembrance of Jesus. Then God, seeing your intellect so strict in guarding itself in every way against the enemy, will Himself bestow pure and unerring vision upon it and will make it participate in God and share in all other blessings.
What is of God, says St Isaac, comes of itself, without you knowing when it will come. Our natural enemy - the demon who operates in the seat of our desiring power - gives the spirit-forces various guises in our imagination. In this way he substitutes his own unruly heat for spiritual warmth, so that the soul is oppressed by this deceit. For spiritual delight he substitutes mindless joy and a muggy sense of pleasure, inducing selfsatisfaction and vanity. Thus he tries to conceal himself from those who lack experience and to persuade them to take his delusions for manifestations of spiritual joy. But time, experience and perspicacity will reveal him to those not entirely ignorant of his wiles. As the palate discriminates between different kinds of food (cf. Eccles. 36:18,19), so the spiritual sense of taste clearly and unerringly reveals everything as it truly is.