my feeling/guess is that demonic possession can occur but it is extremely rare
complete demonic possession occurs after the victim has been spiritually battered for a long period of time; the victim finally submits
I also wonder about protective or guardian beings and their role, while the human victim is being tormented, there's probably a battle going on between these protective beings and the evil ones
during initial stages of being tormented, no paranormal/supernatural phenomena is displayed, so if one complains of being harassed by invisible entities, the cause is often said to be mental illness when in reality it really could be that the persion is being tormented by entities
having said this, I think a lot of people who make such claims of being haunted just need fresh air, sunshine, exercise, a healthy lifestyle, should avoid ingesting supernatural material from books and movies
I guess only a real psychic can tell if a person is truly being bothered by evil entities or not
maybe humans are frequently manipulated in a sublte manner by beings both evil and good
I am reminded of a scene from the movie constantine(which I really liked)
I don't believe in a devil, but my guess/feeling is that demons do exist and everyone is susceptible to their attacks whatever faith they may belong to if any
I think a lot of people who claim to be demonologists or exorcists are in a state of delusion or are conmen, and have no idea how dangerous and serious is the task of battling and exorcising evil entities
just some thoughts, guesswork