1) The film endures because the atmosphere it depicts has become our own. The point—often lost even four decades later—is to convince human beings that we are animalistic and not worthy of God’s love.
In the book, Father Lankester Merrin (explains) the demon’s target is not the innocent girl ... The target “is us.”
Amidst the drugs, scandal, rock and roll, and moral collapse of the 1970s, "The Exorcist" announced ... some evils that are timeless and don’t change.