Oh ye of little faith, read not this book for thine answers.
Damien Karras may have got his answers in the end, but I’m not sure that the rest of us agnostics are sold.
Damien Karras, priest and psychiatrist, is our skeptical eye on the matter of Regan’s possession, and the perfect candidate for the demon’s mind tricks. Karras has suffered a crisis of faith. His psychological training has taught him to view those behaviours previously thought to be caused by possession, as mental defects. The world was full of horrible things and yet there is no word of God’s helping even the most pious and faithful. It is for this reason that Chris MacNeil, Regan’s mother, says that after her experience she might believe in the devil but she still doesn’t believe in God.
It is much easier to believe that there is an external cause for such behaviour as we see Regan exhibit, than to say that it is something within her that caused it. While in this fictitious case Regan was possessed by some external foe, we’ve heard the same theory used to explain away the behaviour of such notorious people as Hitler, Napoleon, and U.S. President George W. Bush. It is a lot easier to say that the devil made them do it rather than have us accept that such horrors could be perpetrated by a person using their own free will.
In the end, as also happens in the movie, they do not drive out the demon through exorcism, but rather because Karras takes the demon into him and then dives out the window. One wonders whether this might have happened to Burke Dennings as well but that man wasn’t enough a person for the Demon.