Who doesn't live in fear of answering the doorbell and being greeted by the beaming faces of Christian missionaries? They stand there rabbiting on, thrusting pamphlets in your face, while you try to coax them off your doorstep without coming across as rude. Such encounters usually end with you having to resort to curtness, informing them that nothing they say is going to turn you into a convert while you shut the door in their faces. Then you have that immediate brief pang of guilt. Maybe you should have invited them in and explained in detail why the conclusions you've personally reached regarding spirituality are incompatible with what they're selling? But who has time for that? And anyway, nothing you say is likely to penetrate their belief system.

Spare a thought for the missionaries though. For the rest of us, such encounters are a couple of minutes every few months. For them it's a daily occupation. Think of the abuse they no doubt regularly receive. It must be like being a door to door vacuum cleaner salesman, if a large portion of the population believed vacuum cleaners were responsible for most of the world's ills.

In horror movies, Christian figures tend to fall into one of two camps: they're either mouth-foaming fundamentalists like Carrie White's mother or priests suffering crises of faith. In reality most Christians are neither raving loons nor racked by doubt. They don't feel the need to either vehemently defend or rigorously interrogate their faith because their beliefs are rarely questioned. In today's over-sensitive world it's considered impolite, offensive even, to ask someone why they have chosen a certain set of beliefs, or to dare to point out the flaws and wrongdoings of a particular sect.

The heroines of Scott Beck and Bryan Woods' (the duo best known for penning A Quiet Place) surprisingly thoughtful theological thriller Heretic are a pair of young Mormon missionaries. Aside from their conservative dress sense, Sister Paxton (Chloe East) and Sister Barnes (Sophie Thatcher, who was raised Mormon before leaving the church to pursue an incompatible acting career) are much like any young girls. When we meet them first they're blushing and giggling about taboo sex topics, Paxton confessing to having once seen a porn film while Barnes questions the claim that Trojan Magnums are really any bigger than regular condoms (this idea of taking a claim at face value will become the movie's core theme). They're by no means fundamentalists, but they have no doubts that they're on the right path.

That path leads them to the doorstep of Mr. Reed (Hugh Grant), who has expressed interest in learning more about their church. Reed lives in a secluded old house, and when the girls arrive it's in the middle of a rain storm (a nod perhaps to the influence of 1977's Death Game, recently remade as Eli Roth's Knock Knock, with its similar "two young women vs middle-aged smartass" dynamic?). Reed offers to give them shelter in his home, but the girls inform him that they can't be alone indoors with a man. Reed assures them that his wife is at home, baking some scrummy blueberry pie no less, and so the girls step inside.

Red flags immediately begin to pop up like a communist coup. Reed asks the girls if they're okay with the walls and ceilings being lined with metal, a question so disarming they don't have time to think about its implications. When the girls nervously ask for his wife to join them, Reed keeps assuring them that she'll show up any second now, pie in hand. His initial theological curiosity takes an uncomfortable turn when he probes the girls for their thoughts on their church's controversial history of polygamy. A candle on the coffee table has the suspiciously coincidental scent of blueberry pie. There's no sign of any forthcoming real pie however, nor of Reed's wife.

Without giving anything away, what develops is a psychological and philosophical game of cat and mouse as Reed's true motives for ensuring a pair of missionaries land on his doorstep gradually become clear. Woods and Beck keep us guessing in this regard, only fully revealing what's at play in the final minutes. The reveal is something of an anti-climax but the journey to that point is gripping.

What's so interesting about Heretic is how it leans into our current culture war, presenting us with protagonists and an antagonist who seem to crudely represent each side of the divide: the secular liberal and the Christian conservatives, and it's refreshing that the former is the villain for a change. For those of us who find both sides equally annoying, there's a lot of fun in watching them dismantle each other's rigid beliefs. But as the narrative progresses we begin to wonder if Reed is really just an atheist blowhard who watched too many Christopher Hitchens clips, or if he's actually far more religious than his young visitors. In a way, Reed represents the 21st century corruption of atheism, which has seen a host of grifters turn a lack of belief into something as dogmatic as any spiritual sect.

Heretic is most compelling in its first half when we're essentially watching a tense theological debate. Woods and Beck avoid falling into the "why isn't this a play?" trap by having their characters' visual expressions contradict their words, a nuance that would be impossible to convey on a stage. Thatcher is especially magnificent at conveying how Barnes, the more worldly and cynical of the girls, figures out Reed has no real interest in what they're selling. We immediately see the resignation on her face as she assumes she's in the presence of another liberal out to play gotcha with her beliefs, which gradually turns to terror as the true horror of the scenario unfolds.

Horror is the one genre that rarely goes out of fashion and in recent times it's taken up the slack left by Hollywood's reluctance to make movies that dare to explore adult themes. Where else are you going to find the sort of philosophical questions asked in Heretic? Horror is now the only place in mainstream cinema where the Big Questions are asked, where the human condition is explored. What's most laudable about Woods and Beck's film is how it may lead some viewers to question their own belief systems, regardless of which side of the fence they're on.

Directed by: Scott Beck, Bryan Woods

Starring: Hugh Grant, Sophie Thatcher, Chloe East, Topher Grace