Father Merrin used to teach archaeology at Marshall College and taught Henry Jones, Jr., better known as Indiana
- We know from exposition in the first film that Jones attended the University of Chicago; Marshall was merely where he taught, much later. Also his instructor was Professor Ravenwood, Marian's father (though it's not implausible for a college student to have multiple instructors even in one field). If we're going to go with Marshall, he'd have to be Indy's coworker, not his tutor. Judging by age that seems more probable.
Father Merrin is a Watcher
The ones from Buffy or the ones from Highlander, take your pick. The so-called "demonic posession" is Reagan suffering from a dissociative episode as a result of sexual abuse. The supernatural elements are due to hysterics.
- Metaphysical theses (I said theses) have been composed on this theory, notably by someone named Rob Eger. According to Word of God, they are hogwash
. Blatty meant for the text to be taken exactly as written: Regan was possessed by a demon, who was investigated by Karras, who then assisted Merrin in the exorcism ritual, and eventually evicted. Blatty went into considerable detail in the book to show exactly how possession can be and is mistaken for dissociation/multiple personality, and how Regan's condition did not fit the medical profile, much as Karras tried to get it to.
