What makes The Exorcist scary for non-Christians?

The mood is a big part, the score and cinematography are second only to Friedkin's direction. Friedkin himself was a secular Jew, and even if he was a religious Jew, Satan in Judaism is nothing more than a district attorney in the celestial court or 'divine council.' Not until the Greek Bible (New Testament) does 'The Devil' exist as the character we know in popular culture, with a big assist from John Milton

As we grow older we see a big part of the horror is to put ourselves in Chris' shoes, seeing your own child go through that while being helpless to do anything about it

But the biggest part is the face, from flashes of Pazuzu to Regan's possession. I'm a (mostly) secular Jew and this movie has always terrified me because of what it does with the face. By 'the face' I mean that part of your mind that registers faces, seeking recognition and to be recognized. Levels of this go from family to community members ('you don't look like you're from around here'), all the way to the national level (there is as yet no fully human face recognized across the globe, that only comes with unity as with the Jewish people, a recognizable countenance)

It's the playing with that part of our minds that makes this movie most effective. Superficially, Regan's possessed face is like a scary monster, nothing new there. It is the distortion itself and not the end product that frightens