From The Screwtape Letters—a fictional work written from a senior demon’s perspective, advising a junior tempter.

Our cause is never more in danger than when a human, no longer desiring, but still 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.

The Screwtape Letters
C. S. Lewis