Regarding influence, Friedkin said EWTN:
“The most spiritual movie I’ve ever seen is ‘Ordet’ by Carl Theodor Dreyer. It shows a literal resurrection, very believable. I watched this movie years before making ‘The Exorcist’. ‘, and I knew because of that movie I could literally do an exorcism. I could show it wasn’t some kind of horror movie. Dreyer approached the subject of miracles in a way. very direct… That’s how I wanted to approach the subject of exorcism. So I thought a lot about that movie while I was shooting.”
That makes perfect sense when you compare the two movies. In “Ordet,” miracles happen after a mother dies after a complicated birth. Johannes, the man who thinks he is Jesus, finds determination in the blind faith of the woman’s young daughter and raises the dead. The resurrection is handled with simple conviction and, after the solemnity of the rest of the film, shines like a ray of divine sunlight through an ominous cloudy scene.
For the script of “The Exorcist,” Blatty removed the scientific alternatives he offered in the novel, leaving Friedkin to present his exorcism in an understandable, non-critical, observational way. ritual with an almost documentary rigor.
Dreyer is not a particularly religious person (through Roger Ebert), but he treats his pious characters with the utmost respect. Friedkin has said that he is a believer, and he instills that belief in his film. Both directors are completely serious in their approach and whenever I watch “Ordet” or “The Exorcist,” my skepticism is suspended and I also feel confident for a few hours.
https://www.slashfilm.com/1048033/william-friedkins-biggest-influenced-on-the-exorcist-was-an-obscure-50s-danish-drama-film/ William Friedkin’s biggest influence on The Exorcist was a little-known ’50s Danish drama film