Assuming for the moment that demon possession exists, there are some things I wonder about. What is in it for the demon? How is it benefitted?
Presumably God wants to rid people of their demon. Why does He wait until someone else says the right words, sometimes only after they have fasted for a while?
With the Gaderene story, why so many demons in one man? One demon is more than enough for one man, so it seems a waste of demonic resources. And when they were cast into the pigs, what was the point of them causing the pigs to perish? The demons would have to find a new home somewhere anyway?