"Heretic" opens with an unusual table-setter: Two young missionaries from The Church of Jesus Christ of Latter-day Saints discuss the curious labeling of certain products, particularly why some are marketed as "large" when they all seem to be the same size. "What else do we believe because of marketing?" one asks the other.

That line will echo through the movie, a stimulating discussion of religion that emerges from a horror movie wrapper. Despite a second-half slide and feeling unbalanced, this rare movie combines lots of squirting blood and elevated discussion of the ancient Egyptian god Horus.

Our two church members – played fiercely by Sophie Thatcher and Chloe East – are wandering around trying to covert souls when they knock on the door of a sweet-looking cottage. Its owner, Mr. Reed, offers a hearty "Good afternoon!" He welcomes them in, brings them drinks and promises a blueberry pie. He's also interested in learning more about the church. So far, so good.

Mr. Reed is, of course, if you've seen the poster, the baddie and he's played by Hugh Grant, who doesn't go the snarling, dead-eyed Hannibal Lecter route in "Heretic." Grant is the slightly bumbling, bashful and self-mocking character we fell in love with in "Four Weddings and a Funeral," but with a smear of menace. He gradually reveals that he actually knows quite a bit about the Mormon religion – and all religions.

"It's good to be religious," he says jauntily and promises his wife will join them soon, a requirement for the church. Homey touches in his home include a framed "Bless This Mess" needlepoint on a wall, but there are also oddities, like his lights are on a timer and there's metal in the walls and ceilings.

Writer-directors Scott Beck and Bryan Wood – who also combined on "A Quiet Place" – have remarkably set us up for an unexpected theological debate here. Mr. Reed is not unlike an earnest professor of comparative religion who is set against two naive missionaries armed with talking points who are hiding their own doubts.

Mr. Reed knows the weak points and thrusts in the philosophical knife. "How do you feel about awkward questions?" he asks before tackling the church's stance on polygamy. "Yeah, it's sketch, for sure," East's Sister Paxton finally admits. Soon, the discussion turns to which religions are marketed better. After all, Mr. Reed is facing a pair of walking and talking advertisements for Mormonism.

So beautifully constructed and acted in the first half is "Heretic" that you won't really notice when it turns into a horror movie. You might be a step ahead of the missionaries, but not by much. Mr. Reed alternates between creepy and funny, well versed in Spider-Man and Voltaire, Radiohead and the Hollies, Wendy's and Taco Bell. Grant has gloriously weaponized his natural charm.

Mr. Reed has his own grand theory about religion and you will learn it. And he may or may not have some creepy stuff in his basement. "It's all terrifying. It is scary. I'm scared," he says adorably, but he's referring to organized religions. You'll find him utterly terrifying, a fanatical heretic in sheep's clothing who can cutely mimic Jar Jar Binks from the Star Wars universe.

Beck and Wood take this fascinating premise as far as it can go before it becomes an airless stage play. By the halfway point, the audience who came for the horror – not the lectures on religious marketing – are baying for blood, and blood they will get. By the end, the plot is a murky, muddled and disturbing mess, a combination of too many ideas and no clear ending.

Grant, with his comfy cardigan and candles, is the movie's draw, but there's great work by Thatcher and East, who are trying not to act scared even when they're terrified. And they're no mere ingenue targets – they bite back with worthy criticism of Mr. Reed's beliefs through shaking teeth.

Producers have added a little marketing manipulation with "Heretic," adding to some screenings the scent of blueberry pie and gore. Don't be distracted. Keep your eyes on Hugh Grant and, well, pray.

"Heretic," an A24 release that opens in theaters Friday, is rated R for "some bloody violence." Running time: 110 minutes. Three stars out of four.