How do you recognize one’s faith?

[The demon Screwtape writes:] 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.
– C. S. Lewis (taken from The Screwtape Letters)