Seven young adults gather for informal Sunday worship in a two-room house fashioned from plastic sheeting and lumber that they cut themselves.

Clad in shorts and jeans and clutching well-thumbed Bibles, they join in song with guitar accompaniment.

“Lord, reign in me, reign in your power, over all my dreams, in my darkest hour.”

The melody drifts across a surrounding makeshift encampment in Jersey Shore, Pa., where 28 students, including a Maine woman, have spent the past two weeks, the final exercise in a training program for the most exotic vocation imaginable.

This is the conclusion of a yearlong boot camp for New Tribes Mission that’s far more rigorous than the usual orientation programs for foreign missionaries – and for good reason.

New Tribes specializes in evangelism among the 3,000 indigenous groups in the world’s remotest tracts, places that remain isolated from the outside world and thus untouched by Christianity. Most operations are in Latin America, Southeast Asia and West Africa.

Teams of five or six missionaries leave the Western world and its conveniences behind to spend years living among tribespeople, learning their language and culture in order to translate the New Testament and parts of the Old Testament into tribal languages, most of which have never been reduced to writing.

The workers then teach reading and writing and establish churches to be run by tribal converts.

Groups may spend 10 or 20 years, or even longer, in the same location. Think of it as career-length “Survivor” – only for real.

“We’re way out there. We’re like the Marines of the church,” says Greg Sanford, the sophisticated but plainspoken director of the Pennsylvania institute.

Despite the rigors and outsiders’ accusations of cultural imperialism, New Tribes, based in Sanford, Fla., has assembled one of the largest missionary forces in the world: 3,200 workers in 17 nations, two-thirds of them Americans.

Enlistees aren’t lured by the money. The mission’s recommended pay for a couple without children is $4,000 a month, before deductions for all benefits and business expenses. Candidates must raise that on their own through pledges from supporters. Some 20,000 U.S. congregations and thousands of individuals contributed $41 million last year, providing most of the mission’s revenue.

The work can be dangerous. During New Tribes’ 62 years of operation, 87 missionaries have died in untimely ways, the vast majority in plane crashes during the early years.

The mission’s very first foray in 1943 ended disastrously when fearful Bolivian tribesmen killed all five visiting missionaries, though contact was later established and today the local people are one-third Christian.

Twenty-two missionaries have been kidnapped, and six killed. The latest victim was Martin Burnham, who was shot to death in 2002 during an attempt to free him from Muslim kidnappers in the Philippines; his wife, Gracia, was wounded. New Tribes recently intensified training in security measures and how to act if taken hostage.

“There are always concerns about safety and different diseases,” says recruit Ruth Dickey from Bowdoinham, who is pregnant with her first child. “You have to overcome fear with the knowledge that the Lord will take care of us.”

Another candidate, Robyn Lenz of Climax, Mich., great-granddaughter of a Bolivian martyr, says institute training built her confidence, proving “you can do without and enjoy it” and “make things very homelike” in the wilderness.

Students are taught how to preserve food, make bread, give a haircut, weld, log, situate and frame a house, collect and treat water, fix plumbing and septic systems, and do maintenance on small engines, solar batteries and portable generators.

“In 90 percent of our countries, you’re on your own,” says Kim Waldon, a former missionary to Papua New Guinea who runs most of the hands-on coursework.

Other institute classes, equally practical, teach time management, mediation of team conflicts, how to maintain morale and solid marriages under stress, and child-rearing in the bush. Missionaries’ children typically receive home schooling for the lower grades, then attend New Tribes boarding schools.

Sanford carefully interviews all incoming candidates. “I weed out as many Indiana Joneses as I can,” he explains, since lust for adventure won’t last for the long haul. Other essential traits: excellent health, teamwork skills and “discipline, commitment, initiative.” Chats with his charges show them to be low key and humble, yet self-assured.

The crucial aspect of the training is more conceptual, teaching how modern missionaries should approach cultures that are radically different. The heart of it, Sanford says, is distinguishing between biblical basics and Western cultural assumptions.

For instance, he spent 14 years with Venezuela’s Yanomamo tribespeople, who did without clothing. Instead of changing that, the missionaries learned the group’s own traditions of modesty. Other practices violated biblical teaching, such as wife-beating and killing newborn twins. The institute trains missionaries to patiently suppress revulsion over such things and realize that changes will only occur after individuals become Christians.

Students spend two years at Bible college before the Missions Institute, and afterward move to the Language Institute in Camdenton, Mo., where they learn new languages and, to fabricate them Pike-style in written form, and translate the Bible.

“It’s a big job. It’s a killer,” Sanford says. One language New Tribes encountered has 14 vowels, another has four forms of “we” and yet another lacks words for “grace” or “salvation.”

Survival International, the London-based tribal-rights champion, and many academic anthropologists criticize incursions by missionaries. Survival supports “tribal peoples’ right to choose their own religions” and opposes missionaries “who force their own beliefs on others.” It says in some cases New Tribes has increased conflict within tribes.

But Sanford vigorously defends New Tribes practices. He insists that the missionaries help preserve tribal cultures rather than undermining them, and are humanitarians who provide literacy, basic medical treatment and other helpful knowledge.

They do, of course, import Christianity to compete with local forms of animism. On that, New Tribes says it provides the opportunity for people to learn about the Bible if they wish, but doesn’t believe in forcing faith upon anyone.

The church relations director at New Tribes headquarters, Dave Zelenak, chuckles at accusations about imposing beliefs: “They must have watched some old Tarzan movie to think somehow you come in and everybody just obeys your word. Not a chance. You have to earn respect.”

At the Sunday camp devotionals, Craig Schafer of St. Johns, Mich., told his six teammates that they might have missed things like ice cream or pizza during their simulated bush experience, but “do we hunger for the Word of God like that?”

If so, he said, taking the biblical message to people who have never heard it is an exciting prospect. “That’s what life is all about. There’s nothing greater in this life.”