The Exorcist is one of those horror movies that I put into a box marked "go into with an open mind and in the context of the time". Blair Witch is another one. Both are technically very good, atmospheric and unsettling films but, unfortunately, due to being overhyped and horror films nowadays (that, for the most part, don't have as many sensitivities to have to combat to get released and pretty much go all out to shock), they seem quite tame.
But clear your head of all the hype, stick the film on, transport yourself back 30 years*, and it's very disturbing.
*Remember that more people were religious then, as well. This is before the Dawkins revolution and when it became fashionable to be atheist. Faith played a much bigger role in everyday culture. So a religious themed horror film was always going to have a greater capacity to unsettle than it does now.