Thou believest that God is one (su pisteuei oti ei qeo estin). James goes on with his reply and takes up mere creed apart from works, belief that God exists (there is one God), a fundamental doctrine, but that is not belief or trust in God. It may be mere creed. Thou doest well (kalw poiei). That is good as far as it goes, which is not far. The demons also believe (kai ta daimonia pisteuousin). They go that far (the same verb pisteuw). They never doubt the fact of God's existence. And shudder (kai prissousin). Present active indicative of prissw, old onomatopoetic verb to bristle up, to shudder, only here in N.T. Like Latin horreo (horror, standing of the hair on end with terror). The demons do more than believe a fact. They shudder at it.