In John Bunyan's classic Pilgrim's Progress, Christian and Hopeful stray from the way and wind up in Doubting Castle, held captive by Giant Despair. For days they are beaten by the giant, and are near to losing all hope. Then, one night Christian remembers that a key called Promise lies hidden in his heart. At Hopeful's urging he takes it out, and finds it opens the chain locks on their feet, the prison door, and the large iron gate leading out of the castle. With Promise, they escape the giant, and soon are back traveling on the way.
When we forget, or fail to contemplate and delight in the promises God has made us in Christ Jesus, we may well fall prey to all sorts of worry, sadness, doubt, and despair. And while we can never avoid adverse circumstances in this broken world, we can escape the chains of doubt and despair they intend to wrap around our minds and hearts.
2Pet. 1.3 His divine power has granted to us all things that pertain to life and godliness, through the knowledge of him who called us to his own glory and excellence, 4 by which he has granted to us his precious and very great promises, so that through them you may become partakers of the divine nature, having escaped from the corruption that is in the world because of sinful desire.