Johnny's lack of faith is that he doesn't invoke the name of God. The devil has God's absolute power when applied to testing humanity. The devil lost because th

Johnny's lack of faith is that he doesn't invoke the name of God. The devil has God's absolute power when applied to testing humanity. The devil lost because they wanted to. By losing he has planted the seeds in Johnny that he can live without God's help. It is possible the author intended something else, but this interpretation is the closest to Catholic doctrine I have seen. IIRC I am remembering it from a priest's comments on the song. "Those who study demonology quickly realize that any power lies with the demons." - Some DND manual.