There may be no absolute rules of screenwriting, but one that comes close is never to have a character say to someone leaving after an argument, “You walk out that door, you don’t come back—you understand me?” Yet that’s precisely the cliché yelled by Lemuel (Walton Goggins), a preacher in a small community somewhere in the Appalachians, to his daughter Mara (Alice Englert) toward the end of Britt Poulton and Dan Madison Savage’s debut film.

But despite that depressing miscalculation, “Them That Follow” is an intrinsically interesting film for one basic reason: Lemuel’s congregation is a fundamentalist sect that applies literally the words of Mark 16:18 “They will take up snakes in their hands, and be unharmed.” The preacher at this little ramshackle church with the white neon cross out front is a snake-handler whose followers, including Mara, also raise the rattlers toward the rafters as a sign of their absolute commitment. Of course it’s a dangerous, potentially fatal, practice, which is why the community is tucked away far from the prying eyes of the law.

Among the congregants are laid-back shepherd Zeke and his ferocious wife Hope (Jim Gaffigan and Olivia Colman), but their son, affable Augie (Thomas Mann) has fallen away from the faith, if he ever had it. He and Mara have been best friends since childhood—a friendship that has deepened significantly over the years, as it turns out. But Lemuel has decided that his daughter’s hand will go to Garrett (Lewis Pullman), an intense young man whose dedication to the church appears to be unwavering.

The love triangle is at the center of what little plot there is. Mara has a secret, and when Augie finds out about it, he returns to the church to prove his faith in the prescribed way. Of course, it does not go well, and his parents and the rest of the congregation believe that only their prayers, combined with his own belief, can save him—a sentiment the suffering Augie does not share: he asks to be taken to a hospital, a sure sign he is not a true believer. Zeke and Hope ultimately reach the same conclusion, but perhaps too late. Meanwhile Garrett learns Mara’s secret, and does not handle the revelation well. It ruptures his relationship with Lemuel, and Mara finally takes a stand for herself; thus that misguided line of his.

“Them That Follows” presents a vision of a deeply insular world most viewers will barely understand, let alone sympathize with. But it doesn’t treat its characters contemptuously, and it boasts some powerful performances, particularly from Colman, who effortlessly dominates every scene she’s in (along with the other actors around her). Goggins, who also played a preacher, though of a very different stripe, on “Justified,” is as intense as ever, and Mann carries off his callow nice-boy persona effectively, as well as a prolonged suffering sequence that gives him the chance to show off a more dramatic side. Pullman makes a suitably unlikable sort. There are plenty of snakes, too, some hoisted into the air, others writhing in the sun or over people’s bodies. Their wranglers did a fine job.

As the central figure around whom the story swirls, Englert registers Mara’s generally submissive attitude effectively, but she’s hampered by the slow, gloomy approach favored by Poulson and Savage, along with their editor Joshua Raymond Lee, which keeps her mostly in so recessive a mode that it’s difficult to read her character emotionally. Kaitlyn Dever, as Mara’s sad, lonely friend Dilly, is rather more expressive. The bleak atmosphere of the stifling community is well captured in Carmen Navis’ drab production design and Brett Jutkiewicz’s slate-gray cinematography, while Garth Stevenson contributes an appropriately brooding score.

“Them That Follow”—a pretty clumsy title, both grammatically and in terms of conveying what the film is about—creates an absorbing portrait of a strange religious world, but stumbles in terms of narrative and pacing.