Moreover, demons came out of many people, shouting, “You are the Son of God!” But he rebuked them and would not allow them to speak, because they knew he was the Messiah. Luke 4:41

            In an odd way, the demons became Jesus’ witnesses, verifying his identity.  But Jesus didn’t want demons to be his spokesmen.  He wanted scripture and truth to be his foundation.  The demons, however, could not help but proclaiming truth as they were defeated.  It was their way of saying “You win, Jesus!”  Remember, one day every knee will bow and every tongue will confess that Jesus is Lord.  That includes demons.  And the reason they will confess then and now is because Jesus wins.  Their proclamation as they exited the humans was the reason why they lost.  If there was a post-exorcism interview, the demon would say, “Well, we gave it all we got, but in the end, Jesus is the son of God, so there wasn’t a lot we could do.”