“Not endorsed by the Governing Body of Jehovah’s Witnesses,” say the closing credits of “Apostasy,” and while you see the practical need for the caveat, it could hardly be a clearer statement of the obvious. The controversial Pittsburgh-founded church does not emerge unbruised from British writer-director Daniel Kokotajlo’s fascinating, quietly battering debut feature, but neither does the wider principle of religious devotion — which isn’t idly or secularly dismissed, but rigorously questioned as to its place on the spectrum between personal faith and institutional obligation.

Born out of Kokotajlo’s own history as a former Jehovah’s Witness, this exquisitely anguished, impeccably acted story of a single mother whose unyielding fidelity to the church tears her in different ways from both her daughters benefits immeasurably from its maker’s unique store of first-hand knowledge. Notwithstanding “Apostasy’s” unassuming scale and style, its genuinely revelatory qualities could make it something of an arthouse conversation piece, following a far-and-wide festival run — begun in Toronto’s Discovery program last month. Casting directors, meanwhile, will be keenly noting the names of the film’s two excellent breakthrough performers, Molly Wright and Sacha Parkinson.

Wright, in particular, must serenely shoulder the bulk of the film’s weight before a sharp, elegant structural fillip at the midpoint that drastically rearranges the human stakes in Kokotajlo’s finely magnified script. As 18-year-old Alex, the dutiful youngest daughter of devout Witness Ivanna (Siobhan Finneran), she’s presented as the wholesome future face of an organization that feels increasingly isolated and out of time in their corner of contemporary Manchester. It seems a symbolically pointed detail that the community’s shabby Kingdom Hall is located by the onramp to a freeway, cars rushing past in long shot as the lives inside stay obstinately stationary.

Yet while Alex does everything Jehovah supposedly requires of her, attending all services and submitting to the passionless courtship of Steven (Robert Emms), an old-before-his-time church Elder, she’s made to feel a constant spiritual deficit due to the life-saving blood transfusion she had at birth — viewed by church authorities as a unholy impurity. Doctors recommend another such procedure to treat her chronic anemia; the church, despite her ill health, advises against it. While Alex follows her mother’s unquestioning compliance, her older sister Luisa (Parkinson) is beginning to have doubts, asserting her independence in ways both deliberate — taking an art class at college, dating a worldly Muslim man — and, in the most crucial instance, unplanned.

From this densely packed premise, Kokotajlo teases out multiple lines of rich inquiry, paying equal attention to the internal and external pressures that are softly pulling this family apart: While Luisa’s growing desire for a “normal” life is a more predictable source of strain, it’s a cruel irony that Alex’s meek desire to belong throws her life dangerously off-balance. Initially a stern supporting (if hardly supportive) presence in her daughters’ respective crises, it’s the hidebound Ivanna who emerges, with each pained dramatic stitch of the story, as its most searchingly conflicted figure: a woman not just torn between love for her church and love for her children, but entirely at a loss as to which love she’s morally expected to put first.

While this is hardly unprecedented subject matter on screen — last year’s Italian feature “Worldly Girl” dealt more broadly with youth rebellion within the Jehovah’s Witness flock — “Apostasy” is rare in its visceral and philosophical intensity. As Kokotajlo’s three principals come to different levels of self-awareness at very different paces, he charts their interior changes against the steadfast tenets of their religion with nerve-nettling scrutiny, as the film morphs into something of a psychological thriller. For some followers, their relationship to Jehovah is very much a matter of life and death.

Kokotajlo achieves this complexity with an impressive lack of rhetorical excess at script level; it’s often the unspoken details in “Apostasy” that rattle us most. The fate or whereabouts of the girls’ father is never addressed, though we eventually glean that Ivanna isn’t a widow: Perhaps, we wonder, her faith has cost her a marriage too. By zeroing in so closely and exclusively on the experiences of three women within the church, however, “Apostasy” notes with cutting precision the male-biased power structures present in so much organized religion — though in evaluating the Elders’ misogyny and morality, it draws a careful, curious line between the theology of the Witnesses and their real-world administration. Vigorous arguments among viewers from all manner of religious standpoints will flare up in the film’s wake; post-screening Q&As promise to be unusually lively.

However stances may vary on this intelligent, incendiary material, it’s hard to see many remaining unmoved by the superb performances of Kokotajlo’s three leads. Wright is most moving as she increasingly disrupts Alex’s surface of seraphic innocence with long, jagged pauses of self-doubt and half-reasoned denial. Parkinson nails a tricky transition, beginning as a hesitant mirror image to her sister before peeling off into her own woman, still more certain of who she doesn’t want to be than who she does. As the freeze-dried Ivanna, whose arc is less emphatic and thus more tragically static than that of either girl, Finneran plays stonily against all obvious angles of sympathy, to ultimately heart-stopping effect.

The actors run it like a relay, passing the narrative’s emotional burden between them. The film’s focus, thanks to Napoleon Stratogiannakis’ tight, alert editing, shifts accordingly — sometimes fluidly so, sometimes with the rude, hard ruptures that life occasionally throws us. Adam Scarth’s subtly remarkable cinematography, for its part, makes an intimate virtue of the Academy ratio, suggesting the rigid demarcations of the characters’ world as it cradles their faces in protective, alternating closeups. That sense of airlessness is compounded by the heavy custard-and-mustard color palette used throughout — significantly broken, at one point, by an ocean of gaudy turquoise in the local nail salon where Ivanna and Alex treat themselves on a day out. In “Apostasy,” such simple outside pleasures aren’t just worldly, but of another world altogether.