Jewish director
I didn't know that William Friedkin, the director of the movie, is a Jewish (in both sides), until he died not so long time ago. I find it intriguing that a Jewish individual directed a Christian exorcism film. In an interview, he mentioned, "I'm a Jew, and that's it. In my heart, I believe completely in the Ten Commandments, but I also believe we are all imperfect and at times we just can't cut it." This is the sole quote I've come across where he discusses his Jewish background. It seems he didn't view his religious identity as a central aspect of his personality, possibly explaining why he chose to direct a film on this topic. a Christian directing a "Jewish film" isn't as problematic for him or her due to Christianity's roots in Judaism and not the other way around.
Personally, I wouldn't have taken on this film. It's not about any opposition to Christianity; rather, it's about the challenge of directing something that conflicts with your own religious beliefs. Additionally, I doubt I could perform at my professional best under such circumstances.
This nugget of information is merely intriguing to me and isn't meant to criticize his decision to direct the film. It's clear that he accomplished an impressive job in his directorial role.