True faith obeys God. This verse differentiates true faith from mere belief. Even the demons believes God exists! They fear, they shudder, they tremble at the knowledge of God. Even the demons knows about salvation through forgiveness of sins because of what Jesus did on the cross. Just knowing the truth is not enough to be saved. True faith obeys God. True faith is living for God. Knowledgeable belief rebels against God. One can know the facts and yet still reject the truth. It is like an immature child who is warned by a parent, hears the parent, understands the words, but thinks he or she knows better and does it one's own way against the warning. This is the very basis of sin itself from the very beginning when Adam and Eve went their own way against the warning. Demons shudder in fear and yet still choose to reject putting their own ability above God. Matthew 12:30 "Whoever is not with me is against me." In Mark 5:7 we read of a demon who meets Jesus and acknowledges Him by name.