I certainly wouldn't invite someone suffering with multiple personality disorder (which is almost certainly the way I personally would interpret a case of "posession by a Demon" to pray *with* me. Neither would I suggest they go on a journey to discover their "Power Animal". In fact - as a lay person - if someone I knew believed they were posessed by a Demon or were evincing behavioural traits which might in the past have been 'diagnosed' as posession by Demons, I'd encourage them to NOT to pray or indulge in ANY kind of introspection that might involve communicating with invisible powers of ANY kind.

I disagree that we are merely using different kinds of language to describe the same stuff. Psychological definintions describing various and vastly differing types of mental disorder are necessary to determine what treatment is applicable. I do think we still have a long way to go in learning about the mind.... An awful long way. But we still have come farther than reducing any kind of symptamology to "Demons" and the treatment of such, by "driving them out"