In our lower stories, we often look at ourselves as if we are underserving. We're not king material. We think, "God can't possibly use me to build his perfect nation because i don't have a religious degree." Or you are not a dynamic speaker. Or you are unemployed. Or your marriage just fell apart. Or you haven't been a follower of Jesus since childhood.
But God still has giants to kill. He still has big plans to accomplish that will take someone like David to get the job done. In the Upper Story, God often uses underdogs to advance his plan. He looks inside the heart to find people he can use. With our willingness and His power, nothing is impossible.