British romantic comedy star Hugh Grant, whom we all know from the films “Four Weddings and a Funeral” and “Notting Hill”, has been having crazy fun lately by blending comedy (“Padington 2”) and evil (“The Undoing”). Grant is malevolent and fiercely intelligent perfection taken to extremes in “Heretic,” which is in theaters now after scoring at the Toronto Film Festival.

Grant starts off as his usual genial self as the mysterious Mr. Reed, exuding plenty of charm when he opens the door to his Colorado mountain home to a pair of young Mormon missionaries, Sister Barnes (Sophie Thatcher) and Sister Paxton (Chloe East).

The young ladies are cautious at first because the rules say they cannot enter any venue without a woman present. “Will my wife do it?” He winks as he closes the door with a menacing click, claiming that the lady is inside making an all-American blueberry pie.

This alone should raise your suspicions. It’s also hard not to notice that there’s a blueberry-scented candle lying around and not a mate in sight. But Reed seems incredibly willing to talk to them about defending or losing their religion. And he gets angry at those who refuse to answer his questions.

Writer-directors Scott Beck and Bryan Woods, who hit the box office jackpot with their script for “A Quiet Place,” know how to create tension and fear. The surprising thing is that “Heretic” is also a fascinating mind-bender that asks deep philosophical questions you won’t see coming.

Mr. Reed doesn’t just toy with the two missionaries, who are not as gullible as they first appear, he forces them to practice what they preach. This requires filmmakers to do more than find new ways to terrorize us with things that pop up at night.

From the first moment Grant reveals his icy steely appearance beneath Mr. Reed’s casual demeanor, the young evangelists are searching for a way out. Their captor says he’ll be happy to let them go with just one warning. To exit, they must first pass the exam by choosing one of two doors: Each door has a name written in chalk: “Faith” or “Disbelief.”

Which door would you choose? “Heretic” can’t wait for viewers to accept the challenge. Reed built walls made of metal to cut off cell service. Even when Elder Kennedy (Topher Grace) arrives from church, he is immediately dismissed. We are on our own.

“Heretic” grips you like a vise as Reed tries to convince two missionaries — Thatcher and East, intriguing contrasts, by the way — that they are enthusiasts for the Mormon payout machine, not true believers. Horror movies rarely go this deep, and “Heretic” ups the sinister bar.

Reed has built a terrifying chamber of horrors in his basement not just to get on our nerves, which he does, but to raise the larger cultural issue about the meaning (if any) of religion in a godless modern world.

I’ll stay away from spoilers for the climactic bloodbath and leave the symbols of blades, poisons, and purifying fires for you to figure out. In particular, Reed finding his personal religion in Radiohead’s “Creep” creates a strange, twisted explosion. However, this provocation is after a bigger game.

Grant will scare the hell out of you in “Heretic.” The dreamy romantics of the past have been replaced by an evil entity eager to send us all to hell. Unlock this fascinating game of cat and mouse at your own risk. Mr. Reed means business. So is the movie.