The Holy Bible is very clear about the reality of demonic possession and demonic oppression.
Most mental health professionals do not accept matters of the spirit as relevant and therefore dismiss spiritual matters as irrelevant or archaic, thus closing the door to its possibility. They then seek to come up with answers within their limited experience that may explain what they are witnessing.
They are related to the same scientific community that denies the reality of God and "Creationism", therefore they have closed off a whole world of possibilities and probabilities.
Within my experience, having studied psychology and privy to matters within the mental health field, most mental health professionals are not worth 10 cents. They are like an automobile mechanic who will not accept certain possibilities even if they are accurate. In fact, when one thinks about it, mental health professionals and auto-mechanics stand to make more money by misdiagnosing the problems of their clients. It's called repeat business.
In my mind, auto-mechanics have a lot more integrity than mental health professionals, because they actually dig for the problem whatever it may be, whereas the mental health expert will only look for the problem in their DSM IV (Diagnostic Statistical Manual - Fourth edition). It is their "bible" and if something isn't listed in it, well, it just doesn't exist as a problem. It is also not uncommon for them to see the same client for many, many years, without ever solving the problem but only dragging it out.
Usually the only thing mental health professionals can offer is mind-altering drugs in hopes of masking the problems they cannot cure or understand. Bad for the patient, but good for Big Pharma.
So, do I believe in possession. Yes, I do. I believe in its reality as much as I believe in the incompetence and malpractice of the mental health profession as a whole.
And, please understand that spiritual problems often manifest themselves in mental and physical ways, so to say that one has a chemical imbalance or whatever the case may be, could be true as a symptom, but may not even touch on the root cause.
Therefore, treating the symptom and not the cause is fruitless and this is why nothing ever seems to be resolved, and why so many people are shuffled around and lost within the system.