When C. S. Lewis’s wife Joy died, he went through a crisis of faith. He wrote a book at the time into which he poured out his questions to God. It was called A Grief Observed. This quote is taken from that book, and is part of what he had to learn through this experience.
You never know how much you really believe anything until its truth or falsehood becomes a matter of life and death to you. It is easy to say you believe a rope to be strong and sound as long as you are merely using it to cord a box. But suppose you had to hang by that rope over a precipice. Wouldn’t you then first discover how much you really trusted it? . . . Only a real risk tests the reality of a belief.