Here is an interesting bit of trivia:
Stellan Skarsgard, who plays the young Father Merrin, is actually older than Max Von Sydow, who plays the old Father Merrin in the original, was when the Exorcist came out in 1973.
The most merciful thing in the world, I think, is the inability of the human mind to correlate all its contents. ... The piecing together of dissociated knowledge will open up such terrifying vistas of reality, and of our frightful position therein, that we shall either go mad from the revelation or flee from the light into the peace and safety of a new dark age.
~ Howard Phillips Lovecraft