Ever since resurrecting the erotic thriller with the respectably lurid 2021 drama “The Voyeurs,” it was clear Sydney Sweeney and director Michael Mohan were interested in second comings. So it was only a matter of time before they’d turn their attention to Christ himself in “Immaculate,” an enjoyable if uneven horror film cut from a slightly different cloth than its religious brethren with a captivating turn from its star as a virginal nun whose pregnancy inspires questions beyond the health of the child, and a hell of an ending.

Refreshingly free of the pseudo-theology that usually bogs down films set inside the church, “Immaculate” simply suggests that My Lady of Sorrows is no place for a young woman. A resting home for older sisters to spend their final days, it seems from the nun (Simona Tabasco) trying to make a break for it in its energetic opening sequence, age doesn’t prevent novitiates from dying there too. Still, Sister Cecilia (Sweeney) is blissfully unaware of any of this after arriving in the Italian countryside from Michigan, and it’s clear she won’t waver from serving the lord as she puts up with overly officious customs officers. “What a waste,” one says to the other in a language the American can’t comprehend, but Mohan and Sweeney are both well aware of what they can get away with by placing the cunning blond bombshell in a habit.

Sister Cecilia is appealing for other reasons to the presiding priest, Father Sal Tedeschi (Álvaro Morte). He tries a little too hard to bond with his recent import, confiding that like her he had an unusual path to My Lady of Sorrows, initially studying biology before devoting himself fully to God. Although religion and science usually don’t mix, they become a novel combination for the Molotov cocktail that writer Andrew Lobel has concocted once Sister Cecilia comes down with morning sickness.

“Immaculate” really takes off in its second half — or as the film announces, her second trimester — once doubts grow inside Cecilia along with her belly. Even before Father Sal and his flock are more brazen in their attempts to stop her from leaving the premises with her miracle on the way, she proves herself more resourceful and less timid than they would believe as the film evolves into a taut survival thriller.

In order to get there, the film inorganically leans heavily on a few well-executed but empty jump scares and unusually loud sound effects to unsettle, along with the threat of these black-hooded figures with red face masks who look cool but whose presence and reach is never entirely addressed. Far more effective is the presentation of the convent, beautifully shot by “Columbus” director of photography Elisha Christian as an endless series of corridors from which Sister Cecilia must eventually make her escape. When you rarely see the same area twice, such an effort seems futile.

After auditioning for the role a decade prior, Sweeney eventually produced “Immaculate” in addition to starring in it. Not only does she have the star power now to get a greenlight for such a project, but the film shows how she can really hold the screen. After dispatching one of her captors, a long take that runs the full gamut of emotions from what she’s done to whether she’s saved herself with her actions with the camera staying on her face is extraordinary, only outdone by a riveting finale where her reactions say it all — and probably spares the film an NC-17 rating.

Benedetta Porcaroli, a standout with her mischievous performance in Carolina Cavalli’s comedy “Amanda,” also makes an impression as Sister Gwen, Sister Cecilia’s only friend at the monastery. She isn’t about to bow down to anyone, and though that’s a lesson that takes time for Sister Cecilia, no small part of the satisfaction of “Immaculate” comes from witnessing someone find faith in herself.