Carranca chronicles the couple’s secret life in “Soul by Soul: The Evangelical Mission to Spread the Gospel to Muslims,” her first book in English. (The publisher is Columbia Global Reports, an imprint directed by Nicholas Lemann, a longtime staff writer for The New Yorker.) Their story may sound like an extreme case of religious fervor, but it’s part of a striking phenomenon: the expansion of the evangelical movement in the Global South, and the growing role that Latin Americans play in it—a development that has received ample attention from academia but not enough from journalism. Carranca’s book arrives to fill that void.

To understand why the couple undertook such a dangerous mission, it’s necessary first to understand the radical religious transformation of Latin America. By the late twentieth century, after four hundred years of Catholic hegemony, the region had begun shifting toward Protestantism, with Pentecostalism—a charismatic evangelical faith that began appearing in various countries by the late nineteenth century—driving most of the growth. According to some counts, by the nineteen-eighties, half of Latin American Protestants were Pentecostals. By 2014, a Pew Research Center survey found that about one in five Latin Americans identified as Protestants, though just one in ten said that they had been born into a Protestant family. “Much of the movement away from Catholicism and toward Protestantism in Latin America has occurred in the span of a single lifetime,” Pew reported.

Carranca traces the origin of the couple’s story to the fourth assembly of the liberal World Council of Churches, in Uppsala, Sweden, in July, 1968. The keynote speaker was to have been Martin Luther King, Jr., but he had been assassinated three months earlier. (The final roster of speakers included James BaldwinMargaret MeadPete Seeger, and President Kenneth Kaunda, of Zambia.) A month after the assembly, Soviet tanks rolled into Prague. In this climate of social and political upheaval, Carranca writes, the W.C.C. advocated for shifting the mission of the church from evangelizing the world to transforming it, even by supporting liberation movements and revolutions.

The backlash was immediate. The Billy Graham Evangelistic Association, led by the hugely popular Southern Baptist minister—who, in his sermons and crusades, presented the Cold War agenda as a fight against evil—doubled down on missions with an anti-Marxist agenda. In 1969, Graham’s association sponsored the first gathering of conservative Latin American evangelicals, in Colombia; in the summer of 1974, it held the First International Congress on World Evangelization, in Lausanne, Switzerland.

Up until then, evangelical work had been led by missions in Europe and the United States, a reflection of demographics—the majority of the world’s Christians lived there—and of colonial power. But, as Carranca writes, by the early nineteen-seventies, “there were more Christians in the Global South than in the North for the first time in more than a millennium.” That shift was evident in Lausanne. A group of Latin American evangelical leaders—from Ecuador, Peru, and Puerto Rico—questioned the role of North Americans who exported “an alien culture” to the rest of the world, and they advocated for a “new missionary era,” in which national leaders in the Global South were empowered.

The change seemed inevitable. Carranca writes that just a hundred and thirty-six Latin American missionaries, all but thirty-one of them Brazilians, had been sent abroad by 1974; over the next thirty years, their numbers grew to the tens of thousands. Today, she notes, almost half of all missionaries come from the Global South.

But, often, American evangelicals have kept control of both the missions’ agenda and their funding. Todd Johnson, the co-director of the Center for the Study of Global Christianity, at Gordon-Conwell Theological Seminary, in Massachusetts, said that “plan after plan after plan on how to reach the world” has come from U.S. church leaders and thinkers, and much of the literature is still produced here.

Luis Palau, an Argentinian whom Christianity Today called ​​“one of Billy Graham’s most prominent successors,” was a typical product of that order. For years, Palau served as Graham’s interpreter when he addressed Spanish-speaking audiences. In 1978, with funding from Graham, Palau created his own association in Oregon. Another Argentinean, Luis Bush—who was raised in Brazil and educated in North Carolina and Texas—became the chairman of the Church Growth Commission within a conservative network known as the Latin American Evangelical Confraternity (conela, in Spanish). In 1989, Bush proposed a program to bring Christianity to “unreached” people in what he called the “10/40 Window,” an area situated between the latitudes of ten and forty degrees north, which, he said, encompassed parts of North Africa, the Middle East, and Asia where much of the world’s poor lived—as well as many Muslims, Hindus, and Buddhists.

This was a vision that seemed to fit American evangelism’s shifting goals. “Since the fall of the Berlin Wall in 1989, the global evangelical movement has increasingly turned its attention from communism to Islam,” Carranca writes. The decade that followed saw a fourfold increase in missions to Islamic countries. Aid groups signed the International Red Cross and Red Crescent Movement code of conduct, which proscribes using aid to further political or religious views. Johnson told me that most of them were “gospel-witness missions”—humanitarian efforts that provided local Muslims with housing, medical care, food, and literacy training, but which didn’t include attempts to convert them. But Carranca cites evidence that several missions were overtly proselytizing.

Then came Al Qaeda’s attacks on U.S. soil. “When anti-Islam rhetoric surged after 9/11, many evangelicals felt that the time had come for a significant push to proselytize to Muslims, who could be questioning the acts of fundamentalist coreligionists in the name of their own faith,” Carranca writes. American missionaries were dispatched to countries bordering Afghanistan and sought to enter the country as humanitarian workers from N.G.O.s. In the process, under U.S. aid support for aid organizations, hundreds of millions of dollars in federal funds fuelled Christian groups working in majority-Muslim countries.

Between November and December, 2002, four American missionaries were killed in Lebanon and Yemen. Luis Bush and other evangelical leaders suggested that missionaries from the Global South should be sent instead. They didn’t “carry the baggage of the colonial era or of the West toward the Muslim world, and they shared the culture and skin color,” Bush told Carranca. A leader of conela, the evangelist Ricardo Luna, put it a slightly different way: “We don’t require as many funds, and we are more culturally relevant to the Middle East. . . . We don’t want to change their culture. We celebrate it.”

By the time Carranca wrote her book, Brazil had become the second-largest dispatcher of missionaries after the U.S., which might explain why the Brazilian couple—Carranca calls them S. P. Luiz and Gis, to protect their identity—joined a group of missionaries headed for Afghanistan, a country in the middle of the 10/40 Window. Their story gives a clear sense of the astonishing determination and growth of the global evangelical movement. It took a few years of preparation before the family could finally settle in Kabul, in 2005. Carranca visited them there several times in 2011 and 2012, chronicling the ordeal they went through and the toll it took on their lives and their children’s. Invading Americans or not, they had come to impose the faith of the occupying forces. Their progress was slow: they managed to convert around seventy people. One of the locals newly baptized by the couple or their colleagues had been rejected by his community because he had collaborated with the Americans; others were women who lived in the neighborhood in extreme poverty, and whom the Brazilians fed and taught to speak English. In 2010, some other missionaries in the group and several converts were exposed by a local TV network. Some managed to flee the country, but others were arrested and tortured, and huge anti-Christian protests took place across the nation. Then, in November, 2014, during a wave of attacks by militant groups against security forces in Kabul, the Taliban attacked the house where the Brazilians’ best friends, a family of South African missionaries, lived and worked, killing the husband and his two teen-age children.

S. P. Luiz, Gis, and their children were able to leave Kabul and resettle in Europe, where Carranca continued to visit them. They were shaken by having been forced to leave their mission. But then Luiz thought of the millions of Muslims fleeing to Europe as a result of the invasion of Afghanistan, the rise of isis, and the war in Syria, among other factors, and the thought triggered a revelation. “I’ve been asking myself: What if God is moving people?” he told Carranca. This refugee crisis looked like a blessing to the evangelicals, who could now reach the Muslims outside of war zones and hostile nations. And that is where this crusade continues, as missionaries from the Global South are redeployed to the ports of arrival for the displaced. ◆