“What does it mean to ‘believe’ to you? James 2:19 ‘You believe that there is one God. Good! Even the demons believe that—and shudder.’ Surely, there is more to belief than mere knowledge.”
To place one’s trust in / have faith in. The demons believe in God’s existence, but they have no faith or trust in Him.
More directly... to follow. The demons know who God is but refuse to follow and worship. We are not merely called to believe as a knowing but to believe as a following all He has commanded... and that is a doing.