Regarding the influence, Friedkin told EWTN:
“The most spiritual movie I’ve ever seen is called ‘Ordet’ by Carl Theodor Dreyer. It shows a literal resurrection, so believable. I saw this movie years before I made ‘The Exorcist,’ and I knew because of that film that I could show a literal exorcism. I could show it literally not as some kind of a horror film. Dreyer approached the theme of the miracle very directly… That is how I wanted to approach the theme of exorcism. So I thought a lot about that film while I was shooting mine.”
That makes total sense when you compare the two films. In “Ordet,” the miracle occurs after a mother dies after complicated childbirth. Johannes, the man who thinks he’s Jesus, finds resolve in the blind faith of the woman’s young daughter and brings the deceased back to life. The resurrection is handled with simple conviction and, after the solemnity of the rest of the film, illuminates like a ray of divine sunlight piercing a foreboding cloudscape.
For “The Exorcist” screenplay, Blatty stripped away the scientific alternatives he offered in the novel, leaving Friedkin to present his exorcism in a plainspoken, uncritical manner, observing the ritual with almost documentary-style rigor.
Dreyer was not a particularly religious man (via Roger Ebert), but he treated his pious characters with the utmost respect. Friedkin has said he is a believer, and he transmits that belief to his film. Both directors are completely earnest in their approach and whenever I watch “Ordet” or “The Exorcist,” my skepticism is suspended and I find myself believing for a few hours, too.