If you recall this post back in September 2006, a detailed discussion began following a Howdy Mail question as to who the actual exorcist is; Father Merrin or Father Karras? The topic even garnered this poll.
We later had this confirmation from a fan who had heard author William Peter Blatty speak about the topic, but we now also have confirmation in the form of a written interview with Blatty thanks to The Exorcist Fansite long-timer Ryan.
The most popular understanding amongst audiences was that Merrin would be the Exorcist, thanks largely to the marketing of the film which uses the tagline œOne hope, the only hope¦ The Exorcist while showing Merrin arriving at the MacNeil house (see original TV spot video posted below, for example). However, Blatty states in this interview that Karras is indeed The Exorcist in his eyes. Blatty states “…the exorcist (of the title) is Karras. And Regan’s possession is the crucible of his struggle for faith, his salvation.” Further discussion on the topic is taking place in the Discussion Forums in this thread.
If anyone was going to dig up comments from Blatty, it’d be Ryan. If you haven’t already, don’t forget to check out his site dedicated to The Ninth Configuration and the body of William Peter Blatty’s work here.