In my opinion this song is about someone who has tried for a while to live without God. His house is built on sand (the devil's promise, unstable, unreliable and untrue) and without God his house cannot stand, hence the cracks in the walls that he tries to ignore. Finally he realizes he needs God to help him before this new lifestyle completely takes over him and buries him beneath the sin that he as a human cannot over come alone. God bless :)