Faith as James speaks about it is NOT a system of belief, but a way of life that consciously draws its sustenance from God and lives for God and is energized by God himself. James forces us to look at a practical example. Suppose a brother or sister is without clothes and daily food. Faith is active. Faith motivates us.
Faith makes a difference.
If we trust in the Living God, we can't stand by and mouth positive platitudes in the face of the poor. We must help warm and feed them. Now James goes one step further by highlighting the difference between bare intellectual assent and living faith. You believe that there is one God. He has a pretty acute awareness of spiritual realities.