THE KEY CALLED PROMISE
In John Bunyan's great allegory, Pilgrim's Progress, the incident is related of how Christian decides to leave the Main Highway and follow another Path which seemed easier. But this Path leads him into the territory of Giant Despair who owns Doubting Castle.
Eventually he is captured by Giant Despair and kept in a dungeon. He is advised to kill himself. The Giant said there was no use trying to keep on with his journey. For the time, it seemed as if Despair had really conquered Christian. But then, Hope, Christian's companion, reminds him of previous victories. So it came about that on Saturday about midnight they began to pray, and continued in prayer until almost morning.
Now a little before it was day, good Christian, as one half-amazed, broke out in passionate speech, "What a fool am I thus to lie in a stinking Dungeon, when I may as well be at liberty. I have a Key in my bosom called Promise that will, I am persuaded, open any lock in Doubting Castle." Then said Hopeful, "That's good news. Good Brother, pluck it out of thy bosom and try." And the prison gates flew open.
(Tan, P. L. (1996). Encyclopedia of 7700 Illustrations: Signs of the Times. Garland, TX: Bible Communications, Inc. From a sermon by Matthew Kratz, The Attack on Hope, 8/27/2011)