I've seen several videos/documentaries featuring (usually self-proclaimed) exorcists, both Catholic and Protestant, who perform "exorcism" ceremonies in which they "cast out demons".
I'm personally a Christian, but I have some scepticism about the validity of these ceremonies, partly because I suspect that most of what's happening is that the pastor is inducing psychological phenomena of hysteria and euphoria in people through psychological conditioning techniques (links to more general scientific research in this area from informed readers would be welcome, btw).
What I find harder to explain is that many of these people, during the exorcism, actually begin speaking and behaving as if the actual "demon" is communicating. I have posted links to videos below where people start speaking to the pastor using complete sentences, laughing maniacally and taunting the pastor, all while supposedly the "demon" is doing the talking. This seems like a really extreme psychological phenomenon to be caused through simple psychological manipulation. I should explain that this happens after the people are subjected to significant psychological suggestion, like the pastor addressing the demon and telling them to "come out", etc.
Is it possible that a person, in the right circumstances, could be induced to believe that they are a different entity, and to communicate as if they were that entity, in a fashion which seems similar to what I (a layman) imagine is the psychological phenomenon labelled as "Dissociative Identity Disorder"?
I should add that I, a layman, am highly sceptical of psychiatry and the validity of labels like DID, but that's probably an unrelated topic.
Here are links to two of the most dramatic examples:
And YouTube titles of other videos:
"You may never see an exorcism this violent. Murders 1,000 years ago still haunt a man today." "John Safran vs God - Exorcism" "The Exorcist Claiming To Cure The Sick Of Their Demons: The Murky Truth Of Modern Exorcisms"