“Why wouldst thou be a breeder of sinners?” spat Hamlet. “Get thee to a nunnery!” Oh, if the Prince of Darkness … er, Denmark, only knew what evil lurks within such walls.

In the first “Omen” movie, the infant Antichrist, Damien — born at 6 a.m. on the sixth day of the sixth month — is given to an American diplomat and his wife to be raised as their own. The adoptive father is told that the boy’s mother died during childbirth, but upon closer investigation (exhuming her grave, marked Maria Scianna), he discovers not a human skeleton but that of a jackal. For nearly half a century, that was practically all the backstory audiences needed for “The Omen” to remain one of the most terrifying movies ever made.

Now comes “The First Omen,” the latest in a frenzy of high-profile prequels fleshing out the origins of long-running franchises. Tapping into another trend, “The Omen” also got the reboot treatment in 2006, though this latest entry proceeds as if said reset never happened. Set in 1971, the atmospheric period piece fits rather neatly into the classic trilogy setup, creatively reverse-engineering the legend of Damien’s birth (the fourth, made-for-TV chapter can also be ignored). Who was his mother, and how did this demon child come to be conceived?

Since horror fans know where things are headed, director Akasha Stevenson and co-writers Tim Smith and Keith Thomas can slyly embed references that achieve full ominousness by association with what’s to come — like the nun who steps off a high ledge after pledging, “It’s all for you,” or the close-call opening scene, which foreshadows how the surviving priest dies in the original film. For most audiences, our imaginations did a freakier job of extrapolating Damien’s provenance than this prequel can manage.

That said, Stevenson’s consistently unsettling and gleefully sacrilegious offering packs its share of legitimate shocks en route to one glaringly obvious “surprise.” Like “Rosemary’s Baby” — the film, along with “The Exorcist,” that paved the way for this Satan-centric saga — “The First Omen” focuses on the female perspective and deals in the darkest sort of pregnancy anxieties. After all, what could be worse than carrying the child of a jackal, or whatever the film’s ungodly parent is supposed to be?

Incidentally, long after Ira Levin wrote “Rosemary’s Baby,” he followed it up with a potboiler called “Son of Rosemary,” in which the assumed Antichrist grows up to be a celebrated humanitarian. On the eve of the millennium, the charismatic 33-year-old convinces the whole world to light celebratory candles, unleashing a toxic substance that wipes out humankind … and then Rosemary wakes up — not at the beginning of Levin’s sequel, but at the start of the first book, effectively invalidating all that has come before. There’s a risk, in extending any popular horror myth, of diluting the impact of the original.

Stevenson takes a respectful approach to the “Omen” series, if not to Catholic traditions or clergy. In addition to reverently acknowledging Jerry Goldsmith’s disconcerting choral score, “The First Omen” brings back Father Brennan (Ralph Ineson), whom we saw so iconically impaled in the 1976 film. While it’s fun to see this blathering loon alive again, the true protagonist is a virginal American novice named Margaret (Nell Tiger Free), who arrives wide-eyed and openhearted at Vizzardeli Orphanage in Rome, incapable of imagining the scope of the conspiracy practiced within.

Run by prune-faced old nuns (led by Sônia Braga) and the kindly but not-to-be-trusted Cardinal Lawrence (Bill Nighy), the centuries-old orphanage serves as an incubator for the Antichrist. Meek at first but increasingly defiant as she discovers her superiors’ sinister intentions, Margaret embodies modern audiences’ changing relationship to the church: She approaches her faith with sincerity, only to discover a total perversion of her values by the institution’s supposed authority figures.

Once Brennan plants the seed of his own paranoia in her, the empathetic new arrival starts to worry about one of the girls, Carlita (Nicole Sorace), who keeps to herself, scribbling demented portraits of her suspicious guardians in coal-black pencil. Apparently, Margaret had a disturbing childhood as well, and sees herself in the feral outsider. So she takes Carlita under her wing, assuring the orphan that it’s not unusual to experience extreme visions — an admission that gives director Stevenson license to spring all kinds of jump scares on audiences, then immediately dismiss them as hallucinations.

One such scene etches itself in our minds, impossible to unsee. Margaret hears screaming and follows the sounds to the infirmary, observing a childbirth so unnatural, even the climactic delivery of Damien can’t help seeming tepid by comparison. It’s a wild, word-of-mouth-worthy gimmick, closer in spirit to classic Italian gialli (from which Stevenson derives much of the film’s jagged, destabilizing style) than to the Richard Donner-directed original. The “Omen” brand should get them in the door, while this graphic gag distinguishes it, guaranteeing the otherwise expendable entry a certain shelf life.

The movie’s going to need it, opening two weeks after the indie “Immaculate,” which also plays on a naive novitiate’s unwitting role in an outlandish pregnancy plot. It’s hard to say which film’s premise is more far-fetched, but if you were to subtract the “Omen” element from “The First Omen,” it’s doubtful that Stevenson’s movie could stand alone, whereas “Immaculate” has been luring audiences on its originality. Do audiences have the appetite for two nefarious nun movies?

Free, who plays Sister Margaret, provides an easily identifiable entry point into the film’s appropriately Goth-looking milieu. Like a young Eva Green, her face can read as innocent one moment and cunning the next, inviting the possibility that there’s far more to the character than meets the eye. Through her, the movie takes a righteous approach vis-à-vis this Catholic sect’s transgressions. Ironically, no church on Earth would sanction something so dastardly; it’s the filmmakers who conceived such a twisted plot, with loose ends clearly intended to spawn additional sequels. You’ve heard of faith-based films. A week after Easter, here’s the faith-debasing alternative.