I went to that movie when it first came out. Absolutely scared the Hell out of me. I never really had nightmares, but after that one, I had them for weeks.
I thought the cure would be to watch the movie a second time, this time looking for special effects and movie making techniques. Instead, it etched deeper into my psyche. The nightmares continued for about a month.
A few years later the movie showed up on TV, so I watched it again. I can't say there is no God or Satan or demons, but at the same time my notions about them all had been dramatically altered, so that I was not in that JW fearful state of mind. Anyway, the movie had no effect on me. None!
I think it makes a difference when you see it in your home on a TV, where the screen is smaller, and you don't get that big theater Sound Surround. (Of course, today's home theater systems now rival the movie theaters, but they weren't around when the Exorcist first aired on TV)
About a month ago this movie showed up on TV again. I watched it once more. I can't believe how much more the movie seems so dated and in some respects even phony. I should mention that I have been doing accounting work for a movie production company for the last seven years. As a result, I have become aware, at a rather intimate level, many of the set designs and special effects they utilize to create realism, especially computer graphics and animation. In the case of the Exorcists I am aware that they used coloured contact lenses to make Linda Blaire look possessed.
It would be an interesting study if someone researched the evolution of the sophistication of movie viewers from the time when they first came out, thru the black and white era, on to colour movies, then advanced Dolby sound, and on to today's powerful special effects, etc. The act of watching movies over a long period of time actually changes us and how we interact with movies. For one thing, we do become desensitized to horror and death and killing, even torture.