Excerpt from The Screwtape Letters

Humans are amphibians—half spirit and half animal. ... As spirits they belong to the eternal world, but as animals they inhabit time. This means that while their spirit can be directed to an eternal object, their bodies, passions, and imaginations are in continual change, for to be in time means to change. Their nearest approach to constancy, therefore, is undulation—the repeated return to a level from which they repeatedly fall back, a series of troughs and peaks.

Now it may surprise you to learn that in His efforts to get permanent possession of a soul, He relies on the troughs even more than on the peaks; some of His special favourites have gone through longer and deeper troughs than anyone else. The reason is this. To us a human is primarily good; our aim is the absorption of its will into ours, the increase of our own area of selfhood at its expense. But the obedience which the Enemy demands of men is quite a different thing. One must face the fact that all the talk about His love for men, and His service being perfect freedom, is not (as one would gladly believe) mere propaganda, but an appalling truth. He really does want to fill the universe with a lot of loathsome little replicas of Himself—creatures, whose life, on its miniature scale, will be qualitatively like His own, not because He has absorbed them but because their wills freely conform to His.

And that is where the troughs come in. You must have often wondered why the Enemy does not make more use of His power to be sensibly present to human souls in any degree He chooses and at any moment. But you now see that the Irresistible and the Indisputable are the two weapons which the very nature of His scheme forbids Him to use. Merely to over-ride a human will (as His felt presence in any but the faintest and most mitigated degree would certainly do) would be for Him useless. ...

He is prepared to do a little overriding at the beginning. He will set them off with communications of His presence which, though faint, seem great to them, with emotional sweetness, and easy conquest over temptation. But He never allows this state of affairs to last long. Sooner or later He withdraws, if not in fact, at least from their conscious experience, all those supports and incentives. He leaves the creature to stand up on its own legs—to carry out from the will alone duties which have lost all relish. It is during such trough periods, much more than during the peak periods, that it is growing into the sort of creature He wants it to be. Hence the prayers offered in the state of dryness are those which please Him best. We can drag our patients along by continual tempting, because we design them only for the table, and the more their will is interfered with the better. He cannot "tempt" to virtue as we do to vice. He wants them to learn to walk and must therefore take away His hand; and if only the will to walk is really there He is pleased even with their stumbles. Do not be deceived, Wormwood. Our cause is never more in danger, than when a human, no longer desiring, but intending, to do our Enemy's will, looks round upon a universe from which every trace of Him seems to have vanished, and asks why he has been forsaken, and still obeys.