John Chau, a twenty-six-year-old American missionary, was killed last month on North Sentinel Island, seven hundred miles off the coast of mainland India. Chau was part of a community of people who do extreme, sometimes undercover missionary work among the five billion people who live within the “10/40 window”—a term coined by a Christian missionary strategist named Luis Bush to describe a rectangular region of Africa and Asia that lies between ten and forty degrees north of the equator and is home to the majority of the world’s Muslims, Hindus, and Buddhists. The region is also known as the Resistant Belt, because many countries there make proselytizing illegal and, in some cases, punishable by death. In the last two years, the Trump Administration has won favor with evangelicals by negotiating the release of American pastors who were arrested while proselytizing in Turkey and North Korea. North Sentinel is home to an indigenous population of between fifty and a hundred hunter-gatherers. The Indian government maintains their isolation in order to preserve their culture and protect them from lethal microbes that outsiders might introduce. It is illegal to visit, and the country’s Navy patrols the surrounding seas to prevent visitors from landing.

When Chau was in high school, he learned about North Sentinel Island through the Joshua Project, an evangelical organization that focusses on reaching the world’s last “unreached” people; he spent most of the next decade preparing to carry the gospel there. He attended a private Christian high school in Washington State, and then Oral Roberts, a conservative Christian university in Oklahoma. He became an accomplished outdoorsman and documented his feats of derring-do—chasing cougars, descending dangerous cliffs, and eating mysterious berries—on a blog called the Rugged Trail. In 2017, he joined All Nations, an organization based in Kansas City, Missouri, that trains missionaries to travel to remote locations. “Pack your bags, come to CPx, and get your world rocked by Jesus!” its Web site reads. At one point during a training, he was blindfolded and taken to a remote location where a group of people pretended to be hostile villagers armed with spears. Dr. Mary Ho, the organization’s leader, has said that the group is saddened by Chau’s death, but that she stands by the group’s core purpose. “We are driven to be part of finishing the Great Commission,” she told Christianity Today, referring to verses from the Book of Matthew in which Jesus orders his apostles to “go and make disciples of all nations.”

In secular circles and among evangelicals, Chau’s death has stirred controversy. Attempting to convert indigenous people to Christianity has long been associated with the dubious enterprise of empire. “Conversionary Protestantism is offensive to many,” Ed Stetzer, the Billy Graham Distinguished Chair of Church, Mission, and Evangelism at Wheaton College, wrote recently, in Christianity Today. “The history of such work is filled with stories of bravery, martyrdom, and positive change—but also filled with mistakes, colonialism, and cultural errors.” Kaitlin Curtice, a thirty-year-old Christian member of the Native American Potawatomi Nation, told me that she sees this moment as an opportunity to examine the legacy of evangelism here in America. “I was once like John Allan Chau,” she said. “I was raised to believe that, with everyone you meet, there’s an opportunity to save them.” But conversion can involve erasing part of a person’s cultural identity by transforming her sense of the sacred. We don’t have to look as far as the 10/40 window to understand how this dynamic can work, she told me; colonists in America destroyed native cultures through conversion.

Chau’s death has raised difficult questions for missionaries who work in remote places. Kenneth Nehrbass, an associate professor of intercultural studies at Biola University, a private Christian university in Southern California, spent a decade, from 2002 to 2012, with his wife and four children on the island of Tanna, in Vanuatu, a Pacific island nation west of Fiji. They were translating the New Testament into a local indigenous language on behalf of a nonprofit called Wycliffe Bible Translators, and alongside a team of people from Vanuatu. Wycliffe only sends its translators to places where they are invited, and the Nehrbasses were also working to preserve the language by recording it alongside their translation work. But some of the indigenous people were resistant to the arrival of outsiders who represented change. “The people we worked with wanted to be left alone and didn’t want their kids to enter the modern world, as it didn’t seem to get them very far,” he told me. Chau’s death had also raised a more practical question: When should missionaries break the law in order to preach the gospel? “Evangelicals are talking about ‘How far are we willing to push the envelope and go against a foreign government?’ ” he said. “We’ve gone against governments since the Reformation. During the Cold War, we really saw people smuggling Bibles into China and Russia as heroes. When a government like India says, ‘No foreigners in this island,’ is this just?”

These questions are familiar to me; I spent seven years travelling along the southern edge of the 10/40 window for my book “The Tenth Parallel,” examining what happens when evangelical Christianity encounters other faiths, predominantly Islam. In places where preaching was illegal, missionaries employed a strategy called “creative access,” which involved working a job as cover while secretly attempting to preach the gospel. In an Iraqi war zone, I met a missionary working undercover as a teacher, and, in northern Sudan, I met one working as an aerobics instructor. In Afghanistan, I met with missionaries who were translating ancient folktales into English. They lived quieter lives than Chau’s. Many were seasoned expats who were hard to differentiate from the humanitarian workers I encountered. (Except the missionaries didn’t seem to be drunk as frequently, or to be having affairs.)

Although some organizations have been known to count souls, many missionaries believe that fulfilling the Great Commission involves simply offering people a choice: to follow Jesus or to reject him. Others believe that their lives serve as example enough to those they'd like to reach. Still, missionary work is frequently dangerous for both the missionary and the convert. In countries including Sudan and Yemen, the penalty for conversion to Christianity is death. Once, I asked Franklin Graham—the son of Billy Graham, and one of Trump’s staunchest evangelical allies—about the ethics of preaching in the 10/40 window when it endangers lives. “So I keep my mouth shut, don’t tell them about what God has done for them, keep them in spiritual darkness, they’ll live out their life, they’ll die, and go to hell,” he said. “Or I tell them about God’s son, and if they receive Christ, then I know that their soul is in his hands. Now, could their life come to an end? Yes. All of our lives are going to come to an end. Some of us just a little sooner than others.”

Chau would likely have agreed. In October, he travelled to Port Blair, the capital of the Andaman Islands and a tourist destination with a population of more than a hundred thousand near North Sentinel, and sequestered himself in a safe house, which he describes in his journals as a kind of self-imposed quarantine, to protect the North Sentinelese from disease. He watched his tan fade and read “Lives of the Three Mrs. Judsons,” a biography that follows the wives of Adoniram Judson, who evangelized in Burma during the early eighteen-hundreds. “I’m in awe of how GREAT our God is,” he wrote in his journal, on November 14th. “Even my heritage points to you—me, an American citizen, part Irish, part Native American (Choctaw) part African, and part Chinese and Southeast Asian—thank you Father for using me, for shaping me and molding me to be your ambassador.”