Thou believest that there is one God; thou doest well: the devils also believe, and tremble.No use in believing that Jesus is the Christ by itself if Satan and his hordes also do, i.e. if it does not change your life. Matthew 25:41 is clear on what the fate of those fallen angels is.
James is distinguishing between mere mental assent and true saving faith.
He is NOT advocating that works are necessary to become or stay saved.
Demons are not saved because Jesus did not die for demons and they are not offered the opportunity to receive salvation, which is why they believe and tremble.