To me that's the things with the Warrens, and with the people they have "helped" Helena. Besides the Warrens, who else told these people they were possessed? Did the people involved actually call on the Warren's for help, or did the Warrens somehow find out about the people, decided they were possessed, and then show up and (try to) convince the person that they were possessed.
And then there is the question of what exactly are the Warren's qualifications. What are their credentials? What gives them the authority to declare themselves experts and can state with confidence that no, this guy did not have mental illness, he was in fact possessed -- without even having met the man.
What makes anyone an expert in any claim? I would say I am an expert in floral design since I was a florist for 15+ years, know every design style, have used almost every technique, read and studied designing, etc. I have extensive background in design itself, not just floral.
Here is what wikipedia says about the title 'expert':An expert ( Audio (US) (help·info)) is someone widely recognized as a reliable source of technique or skill whose faculty for judging or deciding rightly, justly, or wisely is accorded authority and status by their peers or the public in a specific well-distinguished domain. An expert, more generally, is a person with extensive knowledge or ability based on research, experience, or occupation and in a particular area of study. Experts are called in for advice on their respective subject, but they do not always agree on the particulars of a field of study. An expert can be, by virtue of credential, training, education, profession, publication or experience, believed to have special knowledge of a subject beyond that of the average person, sufficient that others may officially (and legally) rely upon the individual's opinion. Historically, an expert was referred to as a sage (Sophos). The individual was usually a profound thinker distinguished for wisdom and sound judgment.
I say the Warrens fall into the 'expert' category with great ease. I also believe people have found relief once the Warrens have been involved and therefor felt 'helped'. It doesn't take much to help people- even just knowing someone supports you in what you are having issues with? That support alone is helpful.