From,  The Screwtape Letters,  by C.S. Lewis. 

Screwtape, a chief tempter, imparts wisdom to his apprentice devil, Wormwood: 

…[T]he prayers offered in a state of dryness are those which please Him best … Do not be deceived, Wormwood. 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. 

From Chapter 8.