The former Dolores Hart, now Mother Prioress of the Abbey of Regina Laudis. (Julie Anderson/HBO)

Few things strike me as more sublime or mysterious — and sometimes illogical — than a woman’s choice to take vows as a contemplative, cloistered nun. Even now, with religious vocations waning, women still go off behind ficus walls or down wooded lanes into quiet convents, to answer an important call, don the old habits and live a disciplined life of prayer and chores.

What little we glimpse of them after that point often radiates calm and enviable joy. Gerontologists have been drawn to convent communities to study some sisters’ remarkable good health — the lower rates of dementia, the benefits of a simple diet, the good bone density. The real answer to their well-being, of course, lies in the intangible.

It’s rare to get a chance to puzzle it out the way we do in “God Is the Bigger Elvis,” Rebecca Cammisa’s moving documentary profile of Dolores Hart, who in 1963 gave up movie stardom and broke off her wedding engagement to join a community of Benedictine nuns at the Abbey of Regina Laudis in western Connecticut.

In a spare 36 minutes, “God Is the Bigger Elvis” (making its TV premiere Thursday night on HBO) is a powerfully demystifying look at the nearly 50 years that Hart, 73, has spent devoting herself to God. She made a rare journey back to Hollywood this year to walk the red carpet with the filmmakers when “God Is the Bigger Elvis” was nominated for best documentary short at the Academy Awards.

Cammisa, the daughter of a former nun, treats her subject with the utmost respect, and Hart returns the favor with an honest clarity. She was 19 when she kissed Elvis Presley in the 1957 film “Loving You.” (His first on-screen kiss; hers, too.) With Grace Kellyesque looks, if not quite the smoldering presence, she co-starred in nine more films, including “Where the Boys Are” and “Come Fly With Me.” While performing in a nine-month run of “The Pleasure of His Company” on Broadway in 1959, Hart, always a devout Catholic, made a brief retreat to Regina Laudis to recuperate.

Something changed her on that trip, forever: A few years later, she joined the sisters and never looked back. (Although is that a copy of the Hollywood Reporter the camera spies amid the clutter on the desk next to her laptop? One assumes she is just a good Screen Actors Guild member, alert to the residuals she — and now her convent — can still collect.)

“God Is the Bigger Elvis” is neither evangelical nor journalistically clinical in its aims, choosing instead to attentively listen. As Mother Prioress (the convent’s second in command), Hart is honest about the doubts and struggles she faced after her split-second decision to call off her wedding and instead become a bride of Christ. More illuminating are the stories from some of the 35 other nuns in the community, each of whom left something behind in search of a purer reward, including Sister John Mary, who left a high-flying life in advertising and politics — and alcoholism. They are open about their daily struggle to recommit to their vows.

It’s quite uplifting, especially in the face of so much depressing news about the Catholic Church these days. In archival footage taken at Regina Laudis, the nuns romp through pastures with cows, play guitars and get hip with youth groups in the 1970s. Nuns will never live down the caricatures and stereotypes mainstream culture has bestowed on them (“How Do You Solve a Problem Like Maria?” etc.), but, quite often, we discover in their lives and words something closer to the gospel truth than the male hierarchy seems able to convey.

In the film’s most moving scene, Hart is visited by Don Robinson, the architect she had planned to marry — a trip he makes once a year. His heartbreak is still evident, but so is his understanding. In her peace, he has gained some, too.

God Is the Bigger Elvis

(36 minutes) airs Thursday at 8 p.m. on HBO.