In the fourteenth century two brothers fought for the right to rule over a dukedom in what is now Belgium. The elder brother's name was Raynald, but he was commonly called "Crassus," a Latin nickname meaning "fat," for he was horribly obese. After a heated battle, Raynald's younger brother Edward led a successful revolt against him and assumed the title of Duke over his lands. But instead of killing Raynald, Edward devised a curious imprisonment. He had a room in the castle built around "Crassus," a room with only one door. The door was not locked, the windows were not barred, and Edward promised Raynald that he could regain his land and his title any time that he wanted to. All he would have to do is leave the room. The obstacle to freedom was not in the doors or the windows, but with Raynald himself. Being grossly overweight, he could not fit through the door, even though it was of near-normal size. All Raynald needed to do was diet down to a smaller size, then walk out a free man, with all he had before his fall. However, his younger brother kept sending him an assortment of tasty foods, and Raynald's desire to be free never won out over his desire to eat. Some would accuse Duke Edward of being cruel to his older brother, but he would simply reply, "My brother is not a prisoner. He may leave when he so wills." But Raynald stayed in that room for ten years, until Edward himself was killed in battle.
This accurately illustrates the experience of many Christians. Jesus set them forever free legally, and they may walk in that freedom from sin whenever they choose. But since they keep yielding their bodily appetites to the service of sin, they live a life of defeat, discouragement, and imprisonment.