The mission church is scarcely more than a shed with open sides. Rusty beams support a roof of corrugated metal, and a wooden lectern, unadorned, serves as the pulpit. No cross rises from the roof or hangs behind the lectern on the blue-painted cement wall; there is no cross anywhere. The house of worship is almost nothing. But it is too much for the missionary Rick Maples. "I want this to be the last church," he said. "This should be the last church built in this section of the valley."

With needles nearly bone white, scrub borders the patch of cleared ground - of coarse sand - that surrounds the church. Cactuses, shoulder-high, grow beside spindly bushes throughout the valley, and the vines and stunted trees are studded with thorns. It is a place, this desiccated land in northern Kenya, where living requires severe tenacity. But it is also a valley of abundance. The country's famous game parks are far to the south, yet here miniature antelope leap over the scrub and monkeys idle at the edge of the Mapleses' backyard. A pair of leopards pranced across the yard one evening last year. At the top of the sporadic acacia trees, whose upper branches form a broad, flat, wispy canopy that looks too delicate to support anything heavier than birds, families of baboons move about, feeding on tiny buds. They seem to float on the flimsy treetops.

Rick, his wife, Carrie, and their two daughters, Meghan and Stephanie, moved to this mission outpost in September 2004. Once, their home was in Danville, Calif., an affluent suburb about 30 miles outside San Francisco. Their house "cost a pile," Rick told me, remembering what he termed "my other life," and Carrie recalled the sunken Jacuzzi and the high ceilings and the curved staircase that they draped with garlands at Christmastime. Rick, who is 43 and whose thick, gray crew cut and slightly round cheeks give him an air of constant buoyancy, was a salesman for a company that marketed combustion engines. Carrie, three years younger, with an angular face and a quieter voice that suggest a different, more private kind of resilience, was a nurse who spent most of her career working with pediatric cancer patients. "We were really happy with our life," he said. "We saw about 25 years ahead, and we were happy with what we saw."

We were talking at the dining table in their mission house, down the path from the church. Both house and church were built by the American missionaries whom the Mapleses have replaced. Shabby but serviceable, the small cinder-block house has running water from a tank that is mounted - along with the church bell - on a metal tower in the front yard. The refrigerator operates on kerosene. In California, the house might belong in a slum; here it is luxurious. It sits just outside Kurungu, a town in name only, near the edge of the desert, much closer to the Ethiopian border than to Kenya's capital, Nairobi, which is a 12-to-14-hour drive away, mostly on dirt roads. Kurungu's three or four shops, dim stalls of dusty shelves, rarely sell more than lard and tea leaves, sugar and salt.

The local tribe, the Samburu, are seminomadic herders of cattle and camels and goats. Scattered throughout the valley and surrounding mountains, they live in manyattas, settlements of huts, about four feet tall at the high points of their sloping roofs, covered in thatch and animal skins. A good portion of the Samburu diet - perhaps most of it - consists of milk and cow's blood, blood drained by cinching a rope tourniquet around the base of the cow's neck, then shooting an arrow into the side of the neck (without killing the animal) and letting the dark liquid spurt into a wooden tankard.

Much of Africa, and certainly much of Kenya, one of the continent's most Westernized countries, hold a mix of the modern and the timeless. But around Kurungu, the modern seems to have barely penetrated. Their wooden bells clacking softly in the still air, the herds graze, tended by the Samburu, whose bodies are draped in wraps of brilliant cloth, whose necks and foreheads are resplendent in beads and burnished metal, whose hair is dyed with red ocher.

To reach the Maples family, I'd flown from Nairobi in a plane hardly bigger than a toy; the pilot, Frank Toews, told me how, as a teenager outside Toronto, he dreamed of flying commercial planes but soon realized that the Lord desired something different. He's now one of 20 pilots in the air wing of the Africa Inland Mission, the century-old, primarily American organization, evangelical and nondenominational, that is known as AIM and that has sent Rick and Carrie to work among the Samburu. The hills below us turned from lush to tan, their jagged contours exposed. Soon Frank pointed out the dirt airstrip next to the Mapleses' house. The plane landed and took off again, and for about a week, other than the Mapleses' Land Rover, that was it for comings and goings of mechanical transport around Kurungu - except for one morning, when another Land Rover jounced along the sand thoroughfare that runs past the airstrip and through the valley and beyond. It carried evangelists from somewhere to the south; they were headed off into the desert to translate the Bible into the language of another tribe.

"For us, this is home," Rick said confidently. Carrie agreed that this was where they belonged, by virtue of their calling to convert the Samburu. "How do they know the truth," she paraphrased from the book of Romans, "unless they are told the truth?" In the long run, Rick said, he had his sights set not only on the area around Kurungu but throughout the territory of the tribe.

Rick and Carrie's daughters didn't seem so sure that this was where they were meant to be. Stephanie, their ash-blond 4-year-old, started to cry right after Meghan said grace at dinner on my first night with the Mapleses a few months ago. "I miss my friends," Stephanie said faintly, having just spent a rare weekend with the kids of a missionary family two hours' drive away. Her head still bowed following grace, her crying was all but tearless, and her voice remained almost mute as she reiterated her loneliness - she came as close as a 4-year-old in a floral print shirt could to being a Stoic. She'd been scared a lot lately too, Carrie said, explaining Stephanie's weeping. The family's dog, Cooper, an irrepressible mutt, had been attacked and nearly blinded by a spitting cobra on the Mapleses' back porch. And Stephanie had been hearing about the two lions that had, over the past month, killed several donkeys and a camel close by. The pair of Samburu guards who keep watch over the house recently chased the lions from the low fence of the family's yard.

Meghan, who is 12 and home-schooled, seemed even less sure than her little sister that Kurungu was where she should be. It wasn't that either girl lacked an intrepid spirit. Meghan proved her ability to adapt eight years earlier, when the Mapleses took on their first mission posting, a two-year assignment that they extended into six. That station, too, was in rural Kenya, near the country's western border, in a rain-forest town called Bonjoge. There Meghan picked up the tribal language, Kalenjin, and befriended a pair of slightly older sisters, trailing them around as they helped their mother fetch water and cook the staple dish, maize. But it was different in Kurungu, which is far more remote. Meghan was struggling with the language, and English wasn't a workable option here as it was in Bonjoge. And she was struggling even to find girls around her own age. When the family visited the valley, she saw none. It seemed they stayed up in the mountains, tending goats till it was time to be circumcised and married. "Sometimes I think I can live without friends, I just don't know," she told me, her golden hair falling past the many layers of beaded necklaces - orange and black, yellow and blue - that encircled her throat and shoulders. AIM missionaries get a year of home assignment for every four years in the field. Meghan went to school back in California for about half of second and fifth grades, and there have been just two shorter trips home. The family's next stay in the States won't come until December 2007. She let only a hint of despair seep into her voice as she went on: "I didn't really hear God talk to me telling me to be a missionary."

Her parents did. Rick and Carrie, whose Baptist church in California is deeply evangelical, spoke of receiving signs, affirmations that they were doing the right thing. Over the past century or more, Kenya has been a highly proselytized country; to go by the broad estimates of the U.S. State Department, 70 percent of Kenya's people now avow themselves Christian, with most of the rest divided between Islam and indigenous faiths. The Samburu, a tribe of about 150,000, worship their God, Ngai. Dispatched by a range of Christian agencies and representing a range of denominations, the missionaries strewn among the Samburu have made little progress. Religious statistics about the tribe are scarce; perhaps 2 to 9 percent are Christian. (Almost none are Muslim.)

Rick and Carrie talked about converting the Samburu in a new way. They envision developing what they call a Samburu-style church. They intend, gradually, to hold more and more Christian services not under a roof but under the acacia trees amid the manyattas. They want the sparsely attended church down the path from their house to be superseded. And they plan to teach the lessons of the Bible not through the preaching of written verses but through an emphasis on expansive storytelling that will fit with the Samburu's oral tradition. Rick said that the first lesson he had to impart, the first truth he had to instill in the people, was "a sense of sin and separation from God" - a separation that could be reconciled only through Jesus. He drew from 1 Corinthians to capture the essence of his message: "I give you Christ and Him crucified."

He and Carrie expect the truth to bring more than religious conversion. Once the people have accepted Jesus, they said, they hope to coax them to judge their traditions by the standards of the Gospel. In this way, they plan to inspire - not impose, they stressed - crucial elements of transformation in the culture. They want to elevate the lot of women, to end the ways women are treated as property. And they want to stop the rite of female circumcision, which Carrie and Meghan witnessed for the first time a few months before I arrived, the razoring out of the clitoris that is almost universally practiced among the Samburu. The Mapleses are in Kurungu, Rick said, because "there is unbelievable need."

sense of humanity's dire need - need that is spiritual, need that is earthly - impels a legion of American Christian missionaries out into the world. Around 120,000 are currently stationed abroad, according to Todd Johnson, director of the Center for the Study of Global Christianity at Gordon-Conwell Theological Seminary in Massachusetts. The legion includes members of mainline and evangelical Protestant denominations; it includes Catholics and Mormons and members of the nondenominational megachurches flourishing throughout the United States. One-fourth, Johnson estimates, are spread over Africa, with another quarter in Latin America, a quarter in Europe, one-sixth in Asia and the rest cast over the islands of Oceania. The 120,000 accounts for only those committed to their distant posts for at least two years; short-term missionaries are harder to track. But, tallying only Protestants, the Billy Graham Center at Wheaton College puts the number who served for between two weeks and a year in 2001 (the most recent figure available) at 346,000. Some Christian emissaries are driven solely to proselytize. Others limit themselves to good deeds, to embodying Christ's message without speaking it aloud. For some, Johnson told me, "if you don't mention Jesus in every other sentence, there's something wrong." For others, "just handing out a cup of water is enough." For most, the work involves both word and water.

In Africa, the continent of greatest earthly need, I had come to know the work of missionaries fairly well before my trip to Kurungu. In Sierra Leone, I spent time with a missionary couple from Grand Rapids, Mich., who had raised their three children in a jungle village. Their work ranged from baptizing converts in a stream to building a gravity-fed system of pipes that would bring safe water to villagers ravaged by disease. In southern Sudan, a land where perhaps two million people were killed by almost a half-century of civil war even before the terrors of Darfur began, I watched a missionary from Vienna, Va., try to create peace between embattled southern clans as a first step toward ending the overarching war between north and south. He oversaw the construction of a huge white tent in the middle of an empty plain. Bargaining with freelance bush pilots, he arranged to fly clan commanders to his meeting ground, to assemble them under his tent. Several hundred ragged militiamen and child soldiers arrived on foot, running across the desolate landscape toward the white canvas. Then the missionary convened his peace conference. He preached gently from the Gospels, and the commanders spoke of the suffering of their people and pledged to quit their fratricidal attacks. If such gatherings could help bring unity and strength across the Christian and traditionalist south, and if his work could, in this way, compel the Muslim north into an accord, the spread of peace would be, the missionary told me, "the most powerful statement of the efficacy of the Christian message."

He wasn't at all alone in the scale of his missionary ambition in Africa. Last year, Rick Warren, the California pastor whose books, "The Purpose Driven Life" and "The Purpose Driven Church," have sold well over 20 million copies and whose Saddleback Valley Community Church has a weekly attendance of 23,000, declared Rwanda the world's "first purpose-driven nation." The country would be a test target for his global plan to eradicate spiritual deprivation along with physical poverty and disease and illiteracy. "God gets the most glory when you tackle the biggest giants," he told Christianity Today magazine. Last summer he sent an advance team of about 50 American evangelicals to meet with Rwandan leaders, and soon, he envisions, hundreds of short-term Saddleback missionaries will fan out across the nation, armed with kits of instruction and resources called "church in a box" and "school in a box" and "clinic in a box" that will help them to rescue the country.

Missionary dreams in Africa have long been outsize. David Livingstone, the Scottish Protestant who first sailed to southern Africa in 1841, yearned both to Christianize vast regions of the continent and to eliminate the Arab slave trade. His explorations of the African interior may have been journeys of white arrogance and may have cleared a route for white imperialism, yet his best-selling travelogues stirred outrage at what he described: "The many skeletons we have seen. . .must be attributed, directly or indirectly, to this trade of hell." Livingstone's expeditions helped to spark missionary interest in sub-Saharan Africa, and by the late 19th century, the Western missionary presence, which began with European naval explorations in the 15th century and which had been confined mostly to the coastlands, spread to the interior. Also during the 19th century, the Protestant missionary force increased until it more or less matched the Catholic deployment. Today, among American missionaries, Protestants far outnumber Catholics, Johnson says, and evangelicals have, since the 1960's, become the dominant strain.

In 1900, around 10 percent of sub-Saharan Africans were Christian. Today the figure is about 70 percent, according to Johnson, with Christians defined as those who profess the faith, though their practice may involve a belief in traditional spirits. This tremendous conversion occurred not only because the missionaries moved inland but also because, more and more during the 20th century, they trained and entrusted African pastors to do the proselytizing. Gradually, African church leadership was encouraged - or became inevitable. Meanwhile, the Scriptures were translated into tribal languages, and increasingly in the later part of the century, missionaries embraced a movement of "contextualization": adapting Christianity to local traditions so that, say, a ritual dance telling a story of victory in battle might be altered and included in Christian worship as a celebration of Christ's victory over death - or so that, in the Mapleses' case, a church building might be replaced by the trees. These days, American missionaries tend to be keenly aware that, as Jonathan Bonk, executive director of the Overseas Ministries Study Center, told me, "God doesn't speak one language" and that Christian worship must take indigenous forms. Rick, who often carries a walking stick of blond wood as the Samburu do, is a kind of pioneer, not only because he has settled his family in a place so far afield but also because he would like to leave aspects of Western worship far behind.

ven beyond conversion, and even beyond abolition, the impact of Western missionaries in Africa has often been immense. When peace was finally brokered between north and south in Sudan in January 2005, much of the credit went to evangelicals like Franklin Graham, Billy Graham's son, who runs the mission organization Samaritan's Purse. He and his staff were well acquainted with the country's devastation, and one of his hospitals had been bombed repeatedly in the south. He put pressure on President Bush to make ending Sudan's conflagration a diplomatic priority.

And when, in his 2003 State of the Union address, Bush called on Congress to devote $15 billion to battle H.I.V./AIDS, it was, in strong part, "a consequence of evangelical concern for Africa," Timothy Shah, a political scientist and senior fellow at the Pew Forum on Religion and Public Life, told me. Shah explained that this concern was generated by missions. "No evangelical church is too small that it doesn't have a significant portion of its budget and identity committed to missions," he said. From their outposts, missionaries send open "prayer letters" - long updates about their lives and requests for prayer that will bless their work - to the congregations that support them. At services and denominational conferences, returning missionaries deliver speeches about all they've seen. "There's this organic process that keeps people informed that's rare in American life," Shah said.

With a president who is acutely attentive to the agendas of evangelical Christians, he added, and with evangelicals making up a majority of the Americans who venture out on missions, this process of education, of information that runs from mission post to stateside congregation, has gained particular importance. "The evangelicals' increasing influence on foreign policy is the elephant in the room," he told me. "It means more focus on a continent that otherwise gets forgotten. You have a politically significant constituency behind humanitarian concerns in Africa in a way that hasn't been the case in many, many years." Shah spoke, too, about the influence of individual mission leaders like Graham and Warren, who recently addressed the Council on Foreign Relations, and like Andrew Natsios, a former vice president of the huge Christian outreach organization World Vision U.S., who served, from the first months of the Bush presidency until a few weeks ago, as the head of the Agency for International Development, the government's division for foreign aid.

None of this means that most missionaries, or even most evangelical missionaries, see themselves as policy advocates. The Mapleses certainly don't. In Kurungu, they rarely talk of world affairs. Their devotion - to meeting the "unbelievable need" of the people - is personal, local, solitary. Yet it is also one tiny part of a powerful religiously driven interaction between America and Africa. And if the Mapleses have their way, their work will transfigure the lives of the Samburu. In a prayer letter last July, e-mailed to the States by satellite phone, Rick and Carrie wrote about the circumcision of Samburu girls: "Everything is cut away that would give them sexual pleasure, all without the aid of anesthetic during the procedure or painkillers afterward. As terrible as it is, it is so ingrained in the culture that all the girls welcome it. Without circumcision, they would never be married."

"Oh," the letter ended, in agony for the tribe, "how desperately they need Jesus."

' 'Two warriors are here, Dad," Meghan said one morning. We had just finished breakfast - cereal bought, like almost all the family's food, on trips to Nairobi made every several weeks, sometimes every three months. Meghan's voice was casual; "warrior" and its Samburu equivalent, moran, are words she uses often.

Rick went out the back door to talk with the moran, young men who'd been initiated as herders of the Samburu's most-prized animals, their cattle, and as soldiers if another tribe chose to attack. (No assaults have come against the Samburu for several years, but the pastoralists of northern Kenya have a long, ongoing history of violent rivalries, and a few months earlier, about 60 miles east of Kurungu, a raid by one herding clan upon another left more than 70 people shot or hacked to death.) In wraps of red and blue, the two moran stood in the Mapleses' yard, leaning slightly on long staffs of pale wood that they held to their sides at identical 45-degree angles. Their bodies were nearly as slender as the staffs.

In the Samburu that he'd learned to speak, haltingly, since arriving in Kurungu, Rick gleaned that an elder in the moran's settlement had fallen ill. Rick was soon navigating the Land Rover along vaguely defined trails through the scrub, with the moran in the back seat. At their manyatta, the old man, too weak to stand, was hoisted into the back of the Land Rover. The nearest clinic, a few spare, clean rooms of concrete, with Kenyan nurses but no doctor, is in the town of South Horr, a half-hour from Kurungu. The clinic is run by a mission of Italian Catholics.

These are the kinds of things that occupy part of Rick and Carrie's days. They drive a crippled girl to be examined by an AIM missionary doctor two hours away. They haul water from their well, in dozens of jerrycans, to manyattas whose sources of water have vanished in recent drought. Rick repairs Samburu machetes with his welding torch.

And because they do these things, the Mapleses are appreciated around Kurungu, Andrea Lekalayo said. Andrea, who learned his English at the Catholic mission's primary school in South Horr, is a young elder; he'd just passed on from being a moran. He led a crew of moran, adorned with beads and with plastic flowers, in digging a drainage ditch for the mission airstrip. Rick and Carrie paid each man about $2 a day - undoubtedly another reason they were appreciated, in this place with almost no cash economy.

The Mapleses were liked too, the moran said, with Andrea translating, for trying to learn their language. They were liked for spending time with the people, for asking lots of questions, for trying to understand their culture, for attending their ceremonies. "Rick," Andrea said, laughing, "he is almost a Samburu." At break time, the ditch diggers sat on the Mapleses' back porch as Meghan served them tea.

But few around Kurungu seemed much interested in their religion. The Samburu faith is monotheistic. It holds its own sacred history in which, I was told, humankind had once been linked to Ngai by a ladder made of leather. Ages ago, a Samburu man, enraged by the death of his herd, cut the ladder, and ever since the people have been disconnected from their deity. Yet when the Samburu spoke to me about Ngai, they evoked not a divinity that is abstract and removed but one that is, though invisible, close at hand, especially on the steep mountains that bound the valley, and most especially on a particular set of ridges and rocky peaks known collectively as Mount Nyiru. This, the tribe's most hallowed mountain, about 9,000 feet high, rises immediately to the west of Kurungu. It looms over the family's backyard. Ngai is up there, taking care of his people. He had granted the Samburu the knowledge of how to survive on cow's blood, Andrea and his crew said. And he was forgiving when the people did wrong. He had placed a spring at the spot where the leather ladder had been cut. The Samburu told me that their religion makes no prediction of a messiah. They didn't seem to feel the need for one.