Far from criticizing the necessity of faith, James distinguished between two kinds of faith. True saving faith will leave a trail of good deeds after it. Salvation is by faith alone, but the faith that saves is never alone. This aberrant view separated faith from works, saying that both can exist independently of the other. It is utterly impossible to see someone's faith without seeing his or her good works.
Godly deeds are the external evidence of true faith within.
James continued his discussion with the objector by commending his orthodox creed about the oneness of God. In verse 19, James turned from praise to pointed rebuke. Demons have faith in this teaching about God, as do both Jews and Christians. In fact, this belief leads them to the point of terror. Their shuddering response to this teaching is more than those who claim that faith in a person's head may never produce deeds in that person's life.