The video, three minutes long and posted just a week ago, is tearing up the Internet, causing appreciative reactions among heterosexual men and some yoga fans and disapproval among yoga purists. Shot in the penthouse of the Mondrian SoHo hotel in New York, it shows a gorgeous young woman performing expert yoga moves wearing only a black lace bra and panties. As she contorts herself, a man dozes in a mussed-up bed.

Yoga, the movie seems to say, gives you all this: a great body, hot sex, serenity and a fantastic apartment with a 360-degree view of lower Manhattan. The implicit message neatly dovetails with the slogan of Equinox Holdings, the fitness chain that funded the video: “It’s not fitness, it’s life.”  More accurately, it’s a fantasy of the life (body, apartment, boyfriend) many of us wish we had.

The yogini in the video is Briohny Smyth, who was raised in Sydney, lived in Thailand as a teen pop star and now teaches yoga at the Equinox gym in West Hollywood. Purists insist that while what she’s doing might require strength and skill, “it’s not yoga,” says Suhag Shukla, managing director of the Hindu America Foundation.

For one thing, real yoga is an exercise of the mind as well as of the body, and as a mental discipline, it emphasizes restraint, including and especially in matters of sex. (“At corporate, they said they wanted arm balances and inversions. I said I’d put together a routine. They said, “Minimal clothing.” I said, ‘Let’s do it!’ ” Smyth remembers. )

What really bothers the purists, though, is that the video contradicts yoga’s true aim, which is spiritual oneness and a unity of mind and body. Instead, it’s another brick in the $6 billion yoga industry — a crass way to sell gym memberships, just as Lululemon sells expensive stretch pants in the name of the transcendence that is the ostensible aim of yoga practice.

The video is “just emblematic of the Western commercialization of yoga,” Shukla says. “You know, the whole purpose of the physical asanas [poses] is to prepare your body to sit still and focus. It’s not about having a cute ass.”

I would suggest, however, that this tension in the West between “corrupt” and “pure” religion is perennial, going back at least to the era before Jesus, when Jewish sects were at war over who, exactly, was sufficiently holy to perform the sacred duties at God’s Temple in Jerusalem.

Spiritual satisfaction and authenticity have always been matters of taste (as well as of time, place and tribe), and those who claim too loudly to have found the “right” path to a higher truth are at a disadvantage for having said so. In America today, there are the Amish and there are televangelists. Some might say the Amish have a “purer” religion, but millions get a thrill and perhaps even existential guidance from the happy platitudes of Joel Osteen.

And of all the religious patterns occurring among Americans now, none is more prevalent than the widespread dissatisfaction with established religion, a falling away of the faithful from the structures and rules of conventional Judeo-Christian worship. In its place is a more do-it-yourself spirituality, a cobbling together of private-prayer, transcendent experience and family tradition; for millions of these DIYers, yoga and meditation meet a need that regular churchgoing can’t fill.

Smyth is one of these new seekers. She believes that yoga changed her life — from a rebellious, anorexic young woman to thriving single mom. She went to Catholic schools as a child, and although she continues to pray to God, “I’ve never felt comfortable with rules,” she says. “The yogic way allows religion to be there as a friend, something you can rely on, not fear.” Her intent in making the video, she says, was only to inspire.

I asked Shukla to suggest an American teacher who practices yoga in a way she believes is consistent with its Hindu roots, and she pointed me to Eddie Stern, a teacher with a studio in an unglamorous building in downtown Manhattan. Inside, he has built a temple — complete with incense and statues of the gods Ganesh, Shiva and Hanuman, to which he gives daily offerings of food and flowers. In the temple, Stern teaches difficult Ashtanga yoga poses, yes, but also chanting, meditation and Hindu philosophy, a fully integrated practice for busy New Yorkers.

Stern has the visage of someone who has discovered the secret to something. He’s monkish but youthful, unwrinkled and lithe. When I asked him whether gym yoga qualified as real yoga, Stern, in his wisdom, declined to condemn.

“It’s hard to whitewash an entire genre of yoga,” he said. “The people who are going to a gym yoga class are going because they hear the word ‘yoga.’ They’re not going to spinning, or aerobics.” In other words, they’re looking for something. “And what we have is really, really good, and powerful and deep. Really, really deep.”

Ironically, Stern’s values jibe more closely with those of religious conservatives than with the shallow purveyors of (Eastern or Western) spirituality prevalent today. He believes that religious practice should be difficult and that nothing worth having, doing or learning is easily attained. By those lights, Briohny Smyth’s performance in the Equinox video is awe-inspiring, regardless of her underwear.