You can't compare Satan and the demons' belief to ours. They were created already saved but they rebelled and lost it. We are born lost and need salvation. It's completely opposite. The demons knew from their beginning exactly who Jesus is but Satan led them astray. Now they tremble because they know what they've thrown away and what's coming in the end.
I think James was speaking in general terms more so as an expert on demonology. I take it as more of a simple admonition to show good works than as him laying down absolutes about salvation requirements and laws governing demons. If you interpret it wrongly to say works are necessary than you have to discount all of Romans as false and many other scriptures. As I've said many times scripture must be interpreted so that it all agrees, otherwise you have misunderstood.
