That movie scared the CRAP out of me, too!
I'm a big skeerdy cat when it comes to satanic and demonic possession movies, but it was only AFTER I saw that movie (and then Beyond the Door a couple of years later).
My mom loved all things like that - she read all the scary books, the True Crime, True Detective mags and such when we lived overseas (no TV!) Those mags described a lot of crimes of gore and stuff. I could never read those, either. I read the Exorcist book after it came out, I seem to recall it was right before the movie was going to be out.
Anyhoo, I was trying to fact check my dates and discovered The Exorcist book's publication date was May 5, 1971 - my birthday.
Wiki said the movie was released in Dec. 1973, but I'm certain I did NOT see it that early. I know that because we did not move back to southern Maryland until April 1974. While looking it up, I saw that the movie was released to only 25 movie theatres at the time in Dec. 1973. It was showing in more theatres sometime that following summer, which would make sense. My mom took me because she wanted to see it, and I had just read the book and wanted to see it, and she didn't want me to go with anyone else. We saw it at the Leonardtown movie theatre (which is now The Rex).
I got up to go to the bathroom, and missed the part where she was stabbing her wimmen parts with the Crucifix. TYJ.
But the movie scared the ever living sh*t out of me and I've never gone to see any of the others that came out after it.