It's been almost 20 years, but Matt Hinton still remembers the first time he heard Sacred Harp singing. He'd read about an event in a folk music flyer, at a rural church up north of his home in Atlanta. "I could hear it before I walked in," he said. "And when I walked in, there were far fewer people in there than I expected. It sounded like 300 people singing, and there were only like 60. I thought, these must be the last people on earth doing this."

They weren't. In fact, this music — some of the oldest in America, nurtured for generations in rural churches across the South — is enjoying a resurgence, thanks to a new documentary by Hinton and an annual event in Anniston: Camp Fasola, an old-fashioned summer camp for singing and learning Sacred Harp, which draws hundreds of people from all over the country.

Sacred Harp is a sound like no other: It is unaccompanied singing, using only the human voice, the God-given instrument, the "sacred harp." The stripped-down harmonies harken back to the Renaissance. The lyrics, which speak of sin and redemption, Christ's death and resurrection, harken back to the Puritans.

The singing is fast, loud and unrelenting. The harmonies buzz, in the air and off the walls. It is vigorous, exhausting and purifying. And everybody sings. The congregation is the choir.

Camp Fasola, which started in 2003, is sponsored by the Sacred Harp Musical Heritage Association, whose president is Jeff Sheppard of Anniston. The group also maintains an in-depth Web site — fasola.org — and publishes a yearly book of singings around the country.

Last year, Camp Fasola drew more than 200 participants from 27 states, Canada and Great Britain.

This year, it's so popular it has been split into two camps. A camp for adults was this past week at Camp McDowell, near Jasper. A camp for youth (age 30 and under) will be June 29-July 3 at Camp Lee.

Sheppard, a self-described "country boy," is part of the tradition's old guard. "I was born and raised with Sacred Harp," he said. That's the way it is for most singers, who learn from their parents, who learned from their grandparents, who learned from their great-grandparents. "It's a handed-down tradition," Sheppard said.

Sheppard met his wife, Shelbie, at a singing, and together they have traveled the country as ambassadors for Sacred Harp.

"Once people become aware that they want to sing, coming down to Camp Fasola is kind of revolutionary," Hinton said. "They learn from people who have been singing it for 60 years. To have access to Jeff Sheppard and Ray Hamrick ... it's the equivalent of somebody who wants to play blues guitar hooking up with Muddy Waters and Willie Dixon."

Also driving the renewed interest in Sacred Harp is the documentary Awake My Soul: The Story of the Sacred Harp, made by Hinton and his wife Erica who, — at ages 35 and 34, respectively — are part of the tradition's new guard.

Matt Hinton, who teaches religion at Morehouse College in Atlanta, will be teaching theology classes at Camp Fasola.

Awake My Soul includes footage from singings in Georgia and Alabama. It received widespread play last year on PBS stations and is still being screened at festivals, including one in Brussels, Belgium, in the fall.

Accompanying the documentary are two CDs: a soundtrack of traditional singings and a collection of various artists performing Sacred Harp songs, including Tim Eriksen, John Wesley Harding, Doc Watson and John Paul Jones (of Led Zeppelin).

It's the rare project to get reviewed in both Spin and Christianity Today. (For more info, including how to order a DVD and CDs, visit www.awakemysoul.com.)

Sacred Harp tends to be popular with fans of Americana music, folk — and punk rock, which, like Sacred Harp, is about stripping things down to the basics, about authenticity, about passion.

The new fans, however, don't always understand the theology of the lyrics. "Kids who don't go to church, people who were not raised going to church, for them Sacred Harp offers a sense of authenticity, a sense of community, a sense of tradition, a sense of weight to a life that might otherwise feel weightless," said Hinton.

Sacred Harp singing arrived in America with the colonists. Its simple harmonies are based on four notes: fa, sol, la and mi. Each note is assigned a different shape — triangle, oval, square or diamond. "It was a simple way to teach simple people how to sing," said Sheppard. (That's why Sacred Harp singing is also called "shape-note singing.")

By the 1800s, hundreds of songbooks had been produced. The most famous was published in 1844, compiled by B.F. White of Hamilton, Ga. It was a mix of old and new hymns he called The Sacred Harp. It has been continuously updated, and composers are still adding new songs.

The most famous of the more than 500 hymns in the book goes by its old name, New Britain. Most people know it as Amazing Grace.

As modern choral teachings made their way into church, shape-note singing fell out of favor. But it has been preserved, not in churches, but in weekend singings.

Held in old, rural churches, with no air conditioning, singers can go through 90 or 100 songs a day. They break for a potluck dinner on the grounds.

The singers sit in sections — treble, alto, tenor, bass — arranged in a hollow square. The leader stands in the center, arm pumping to keep time. Nearly everybody takes a turn as leader.

Sacred Harp singing was rediscovered in the 1960s by folklorists like Alan Lomax, and a generation of Southern singers traveled the country, reintroducing this oldest of music. Today there are established groups as far afield as New York City, Chicago, Massachusetts and Portland.

Sacred Harp continues to be nurtured by new generations of singers here in the South, notably the Ivey and the Wootten families of Sand Mountain, Ala.

Hinton is also contemplating new projects. He's been approached about working with previously unreleased footage of singings filmed by Lomax in the 1980s in Alabama and Georgia. There's also talk of another double album.

Even so, there is always the worry that the tradition will die out. "I'd be surprised if there were more than 15 kids in all of Georgia who were really active Sacred Harp singers," said Hinton, "which is not very many for the entire state where it came from."

3 ways to hear (and sing) Sacred Harp

• Camp Fasola, an annual summer camp for learning and singing Sacred Harp, will be June 29-July 3 at Camp Lee in Anniston. For more information and to register, visit fasola.org/camp, or call (404) 237-1246.

• The camp will close with an Independence Day Singing (visitors welcome) at 9 a.m. on July 3. Lunch will be available for purchase in the Camp Lee dining hall.

• In addition, Jeff Sheppard, president of the Sacred Harp Musical Heritage Association, and other local singers gather once a month at the Golden Springs Community Center, from 6:30-8 p.m. the second Tuesday of every month. Newcomers are more than welcome.