Screwtape Letter 9 – More on Undulation, by C.S. Lewis

But there is an even better way of exploiting the trough; I mean through the patients own thoughts about it. As always, the first step is to keep knowledge out of his mind. Do not let him suspect the law of undulation. Let him assume that the first ardours of his conversion might have been expected to law, and ought to have lasted, for ever, and this his present dryness is an equally permanent condition. Having once got this misconception well fixed in his head, you may then proceed in various ways. It all depends on weather your man is of the desponding type who can be tempted to despair, or the wishful-thinking type who can be assured that all is well.