Appparently Wiliam Peter Blatty worked for Psy-Ops, which is Psychological Warfare in the Government.

Given the deep insight into psychiatry that he shows us in the first movie I wouldn't be surprised, and from my experiences from visiting actual mental hospitals.

Another theory is that he works for the "illuminati" if you count all the Crowley images pasted onto the wall of the psychiatrist in Exorcist III, that he directed.