"two demon possessed men coming from the tombs met him. They were so violent that no one could pass that way. 'What do you want with us, Son of God?' they shouted. 'Have you come here to torture us before the appointed time?'" NIV translation
When they arrived at Hippos, they were met by two demon possessed men who were coming from the area of the tombs. At this time, people were placed inside of caves in a hill after they died and this is the place where these men stayed. So what does it mean to be "demon possessed"? That means that an impure spirit is inside of these men for the purpose of torturing them and destroying them because they were created in God's image. The demons would operate by causing mental problems, violence, diseases of the body, and revolt against the ways of God.
Here, we see that these demons demonstrated their presence through violence as they kept people from getting near the area.
We also see that they immediately recognized Jesus for who he is as they addressed Him as the Son of God. This reminds us that even Satan and his followers know who Jesus is and they know of His authority.
In our world today, there are many "religious people" that deny the fact that Jesus is the Son of God. How can this be when even demons accept that fact? The simple fact of the matter is that they have been blinded by a philosophy and believe the lies of Satan and his demons.
You see, the demons know who He is but do not want men to see Him for who He is and the power that He has been given by God. Once people recognize Jesus for who He is, what he has done, and accept Him as their Lord, then, they have the same power and authority that was given to their Master.
The third thing that we see is the fact that the demons knew that He had authority over all things and that there is a time coming where they will be sent to their torture in hell. Even though they recognized Jesus' authority, they wanted men to be blinded to that authority and to accept the lie that all "religions/philosophies" have equal merit. This was the way of the Greeks and it was a characteristic of that area during Jesus' time on earth but has spread to most of the world today.