From the Screwtape Letters (letter 7):
“I wonder you should ask me whether it is essential to keep the patient in ignorance of your own existence. That question, at least for the present phase of the struggle, has been answered for us by the High Command. Our policy, for the moment, is to conceal ourselves. Of course this has not always been so. We are really faced with a cruel dilemma. When the humans disbelieve in our existence we lose all the pleasing results of direct terrorism, and we make no magicians. On the other hand, when they believe in us, we cannot make them materialists and skeptics.
At least, not yet. I have great hopes that we shall learn in due time how to emotionalize and mythologize their science to such an extent that what is, in effect. a belief in us (though not under that name) will creep in while the human mind remains closed to belief in the enemy. The “Life Force,” the worship of sex, and some aspects of Psychoanalysis may here prove useful. If once we can produce our perfect work—the Materialist Magician, the man, not using, but veritably worshiping, what he vaguely calls “Forces” while denying the existence of “spirits”—then the end of the war will be in sight. But in the meantime we must obey our orders. I do not think you will have much difficulty in keeping the patient in the dark.”