William Peter Blatty was pounding out the first pages of "The Exorcist" when his telephone rang -- bringing the news that his mother had died.

The screenwriter was already digging into dark material that was completely different from the whimsical work -- such as the classic "A Shot in the Dark" Pink Panther script -- that established his Hollywood career. He was writing a fictional take on an exorcism case he heard discussed during his Georgetown University studies.

But the death of Blatty's Lebanese-born, fervently Catholic mother changed everything. She spoke very little English and called her son "Il Waheed," Arabic for "the one" or "the only." He struggled with grief for five years and his supernatural thriller turned into something much more ambitious.

"I wanted to write about good and evil and the unseen world all around us. I wanted to make a statement that the grave is not the end, that there is more to life than death," said Blatty, meeting in a diner near the Georgetown neighborhood described in "The Exorcist."

It was 2013, four years before Blatty's death, and our conversation focused on the 40th anniversary of the film that brought him an Academy Award, for adopting his novel for the big screen. Now, on the 50th anniversary of "The Exorcist," critics are still debating why it had such as seismic impact.

Blatty insisted, many times, that he wasn't trying to shock people, even though the R-rated classic sent many rushing for theater exits, sickened by its stomach-wrenching visions. His goal was "apostolic, from the beginning," an attempt to inspire faith and defend core Christian doctrines, he said.

The equation was simple: "If demons are real, why not angels? If angels are real, why not souls? And if souls are real, what about your own soul? … And, by the way, if incarnate evil is real, what are you going to do about that?"

"The Exorcist" set box-office records for horror films, with numbers that soared with subsequent re-releases. At the same time, Blatty was deeply satisfied to hear priests report that, in the weeks after the movie opened, penitents lined up for confession.

Looking back, it's important -- from a Catholic point of view -- to consider the timing for "The Exorcist," said Rod Bennett, author of "The Popcorn Cathedral" and creator of a YouTube channel with the same name. The early 1970s were just after the Second Vatican Council and its efforts to reform, or some would say "modernize" the Catholicism.

"This was a time when lots of modern Catholics were taking a kind of all-bets-are-off attitude. They were saying, 'Everything's different, everything's changed, and you can release all your inhibitions and still be Catholic," said Bennett, a convert to Roman Catholicism.

Then along came "The Exorcist." Those with open eyes could see that "Blatty was poking people really hard, especially Catholics. He was telling anyone who had been lulled to sleep that, no matter what they had heard, the Devil was still out there."

As a genre, horror films seem to have an unusual ability to raise spiritual questions, perhaps because it's easier to craft images of incarnate evil than similar takes on what is eternally holy and good, noted film critic Steven Greydanus of DecentFilms.com, who is a permanent deacon in the Archdiocese of Newark.

The Christian apologist C.S. Lewis, for example, wrote "The Screwtape Letters," a collection of letters from a senior official in hell to a young demon in training. "Who can imagine anyone daring to write a similar collection of letters from an angel? It's hard to imagine anyone even trying to assume that point of view," said Greydanus. "What would you say?"

Meanwhile, opinion polls consistently demonstrate a modern paradox -- with more people saying that they believe in God, than believe in the Devil. Also, many modern believers accept the existence of heaven, but not the reality of hell.

"This is a problem that Blatty seems to have anticipated," said Greydanus. "It seems that what he was doing was arguing backwards from the existence of evil to the existence of God. He decided to come at faith through the back door, so to speak. He scared lots of people, but he also made them think about things they may not have wanted to think about."