In 1678 John Bunyan wrote an allegory about a man by the name of Christian. We know the story by the title, "The Pilgrims Progress." The story is about a man who seeks to find the way to the "celestial city," or heaven. On his way he goes through many difficulties, trials and temptations.
At one point he is caught trespassing on a giant's property. The giant's name was Giant Despair and
his home was called doubting castle. Christian, for trespassing, is thrown into the dungeon of Giant Despair. While in this dungeon Christian and his friend named Hopeful were struggling deeply with depression and were contemplating suicide. Day after day they were being beaten by Giant Despair. Finally when Christian and his companion were about to the point of taking their own lives, Christian cries out, "What a fool. . . am I, thus to lie in a stinking dungeon, when I may as well walk at liberty! I have a key in my bosom, called Promise, that will, I am persuaded, open any lock in Doubting Castle"
That "key of promise" is the word of God. God has promises sprinkled like gems throughout the Bible that we can claim and through faith He will give us the victory. In the allegory Christian is let out of the prison of doubt, depression and despair by trusting in the promises of God's Word. God's Word and His promises are the keys to victory in our own lives.
You see, time and again, in the Old Testament the people of the Lord claiming God's word when they were seeking His mercy. We see in Exodus 34:6 God revealing His character to Moses when He said in Exodus 34:6, "And the LORD passed by before him , and proclaimed , The LORD , The LORD God , merciful and gracious , longsuffering , and abundant in goodness and truth. . ." Then throughout the Old testament God's people remind Him of what He had said in Exodus 34. They were seeking for mercy and they knew that in His Word It had revealed that He was a merciful God, and they would remind Him of what He revealed to Moses. In the same way we can claim God's promises.
A simple example of claiming God's promises would be, like the Israelites we can say, "God you said you are merciful in the Old Testament. You also reveal in I John 1:9, 'If we confess our sins, he is faithful and just to forgive us our sins, and to cleanse us from all unrighteousness.' Lord I have sinned and I am seeking your forgiveness. You also promised to give me forgiveness and you also promised to cleanse me from all unrighteousness and I trust that you will fulfill your promise."