This is gonna be long sorry... Just to be fair this movie is my favorite horror movie so I may be bias. The Exorcist is not just a horror movie its a master class in filmmaking.
To me the realism is due to the fact it was shot like a documentary. And the acting was so incredible that the actors pull you in. To me the slow burn creates such tension and the lack of soundtrack makes the big moments even more effective. The sound design is next level. The makeup and special affects are amazing especially now when you realize it came out in 1973. The shocking and disturbing moments of the film exhaust you mentally to the point that you feel the pain of the actors and become completely invested. I just saw a reactor cry out of concern for Regan and Chris. The despair is overwhelming.
Now to me the meaning of the movie/book are about faith and love.  Karras losing his faith and showing the love he has for his mother and the pain grief he goes through. Chris's love for her daughter and desperation then gaining faith when she never had any. The strength of Merins faith and devotion to God.
Its about the power of God and renewing your faith and knowing his love even when faced with true evil.

This quote is in the book and director's cut of the movie. It sums it up perfectly.  

Father Karras: "Why her? Why this girl, it makes no sense.
Father Merrin: I think the point is to make us despair. To see ourselves as... animal and ugly. To make us reject the possibility that God could love us." That is what William Peter Blatty explained in an interview. I suggest reading the book its a fantastic read. I had the privilege to see this in the theater twice 2000 & for the 50th anniversary it is truly an incredible experience to watch on the big screen, there's nothing like it. 50 years later it still hits hard and will  continue to be the most influential horror film in history.

the carotid angiogram was absolutely the worst for me lol i cant stand watching medical procedures
57:40 iirc hitchcock was super adamant about the twist not being spoiled, and psycho was the reason we have movie show times now, before you could walk in and out as movies continuously played

can’t remember if i’ve seen the scary meter in the past cause i “watch” in my pocket at work but i love the little animation