Answer: The Apostle James says, "You believe that there is one God; you do well: the devils also believe and they tremble." - James 2:19.
The devils know that Messiah Jesus has come and He rose from the dead, like we do. But are the devils saved? Of course not! Yet, there are many people who likewise believe the Gospel, but they are not much better off than a believing demon unless they truly repent; that is, confess their sins, ask for forgiveness and be willing to turn from their sins. This is the one thing a devil cannot do, he cannot repent.
The devils believe and they tremble, a true Christian believes, repents and is saved.
Repentance is what distinguishes a genuine believer from a believing demon.