Seven young adults gather for informal Sunday worship in a rude, 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 to 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 where 28 students have spent the last two weeks, the final exercise in an unusual training program for the most exotic vocation imaginable.

This is the Missions Institute of New Tribes Mission. It is a yearlong boot camp that is 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 modern world and its conveniences to spend years living among tribes, learning their language and culture to translate the New Testament and parts of the Old Testament into tribal languages, most of which have never before 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.

"We're way out there. We're like the Marines of the church," said Greg Sanford, director of the Pennsylvania institute (there are also campuses in Durant, Miss., and Baker City, Ore.).

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.

New Tribes is similar to the even larger Wycliffe Bible Translators, based in nearby Orlando, and the two agencies often cooperate in the field. Both are staunchly evangelical Protestant and employ techniques pioneered by the late University of Michigan anthropologist Kenneth L. Pike to render oral languages into newly written form.

Money is not a lure for enlistees; 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. About 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 first foray in 1943 ended disastrously. Fearful Bolivian tribesmen killed all five visiting missionaries, although contact was later established and today the local people are one-third Christian.

Twenty-two missionaries have been kidnapped, with 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," said recruit Ruth Dickey of Bowdoinham, Maine, 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, said institute training built her confidence, proving that "you can do without and enjoy it" and "make things very homelike" in the wilderness.

Students are taught food preservation, breadmaking, hair cutting, welding, logging, how to situate and frame a house, how to collect and treat water, and the mysteries of plumbing, septic systems, small-engine maintenance, solar batteries and portable generators.

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

Other classes 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 interviews all incoming candidates. "I weed out as many Indiana Joneses as I can," he explains, since lust for adventure will not 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 said, is distinguishing between biblical basics and Western cultural assumptions.

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

"It's a big job. It's a killer," Sanford said. One language that 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 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 mission offers a conservative creed, including the belief that "every person is responsible to receive salvation by personal faith in the Lord Jesus Christ" and that those who do not face "unending punishment." Another specific: New Tribes practices only "believers' baptism by immersion."

While New Tribes' mission has not changed much since it was founded by Paul Fleming, a one-time missionary to British Malaya, technology has aided its work dramatically in recent years. The time required from the original contact with a tribe through completion of a Bible translation used to be 20 to 30 years. Now, thanks mainly to computers, that has been cut in half.

But New Tribes is facing a shrinking number of Americans willing to volunteer. New Tribes was annually sending out about 200 new missionaries in the 1980s; this year, the number is 80.

Meanwhile, missionaries who joined during boom years are retiring and the number of dropouts from the field, though small (eight families in the past three months), is increasing.

Dave Zelenak, the church relations director at New Tribes headquarters, and Paul Wyma, the member of the governing Executive Committee who supervises U.S. operations, cite a number of recruiting problems.

For some churches, the work of missionaries is seen as less urgent these days. For some parents, security worries lead them to discourage children from working overseas.

There is also the lure of contemporary culture as well as a declining number of young Americans who have rural backgrounds or experience with outdoor activity and hard physical labor. For many, spending decades on a narrow-gauge task seems unpalatable.

But recruits such as institute graduate Timothy Depue keep coming.

"The culture has a huge emphasis on material wealth I struggle with," the bearded Depue admits as he cleans up after a simple camp breakfast. "I want the sports bike and the big house. I have to contemplate every day that Jesus gave up all to do what he did."

At the Sunday camp devotionals, Craig Schafer told his six teammates that they might have missed such things as 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."

Craig Schafer of Saint Jones, Mich., studies his Bible at a table in a hut at the Missions Institute. The group has 3,200 workers operating in 17 nations."We're like the Marines of the church," Greg Sanford, right, director of the Pennsylvania institute, said of the missionary organization's purpose.Students such as Karen Gainer spend a year at the Missions Institute before going abroad. Missionaries in training hold a worship service at the Missions Institute of New Tribes Mission in Jersey Shore, Pa.