The people taken hostage in Afghanistan were on a popular kind of tour in which church groups go on short, nonevangelical aid trips. Mostly in their 20s and 30s, all of the hostages were members of the Saemmul Community Church, a Presbyterian congregation in Bundang, a suburb of Seoul. Many were English teachers or medical professionals. Over a 10-day period, the group was scheduled to be in northern Afghanistan, then travel to Kandahar, to organize "medical activities and activities for children," according to Kim Hyung-suk, president of the Korean Foundation for World Aid, which organized the trip.

The Koreans were seized at gunpoint while riding a bus on the highway from Kabul to Kandahar. Their captors have demanded the release of 23 Taliban prisoners in exchange for the hostages, but the Afghan government has refused.

Missionaries in Asia have long faced violence. A hundred years ago, American and European Christians streamed into the region to convert the Chinese and Koreans. During the Boxer Rebellion (1899-1901) in China, foreign missionaries were targeted and in many cases killed. But they kept coming because Asia houses some of the world's largest non-Christian populations. Today, Christians in Asia number 350 million, up from about 20 million in 1900, according to statistics from the Center for the Study of Global Christianity. And as Christianity flourishes, more and more believers -- often Asian -- begin to heed Jesus' instruction to his disciples to spread their faith across the world.

The presence of South Korean Christian aid workers is one of the most visible examples of the trend toward "majority world" missionaries—those hailing from continents other than Europe and North America. South Korea, for example, sent only 93 missionaries abroad in 1979, but by 2000 there were over 8,000 and this number doubled by 2006.

South Korea's fervor is unique in that it's a relatively new Christian nation. The example set by the missionaries (mostly American and British) who came to work in Korea is still a recent memory. Like its neighbors China and Japan, the Korean peninsula was traditionally influenced by Buddhism and Confucianism. A small number of Catholic missionaries came in the late 18th century; their Protestant counterparts arrived about 100 years later. But it wasn't until the 1960s that the number of Christians began to increase dramatically. The traumas of the Japanese occupation (1910-45) and the Korean War (1950-53) had left the country reeling, and some see Christianity's growth as a response to those difficult times.

Although about half of Korean missionaries go to other East Asian countries, a growing number settle in places like Jordan, Turkey and Syria. Korean missionaries were present in Iraq until the 2004 beheading of a Korean translator there. The flow of majority-world missionaries goes from west to east as well: One underground church in China is run by a missionary who felt called to go there from his home in Nigeria.

As the missions increase in size and scope, so do the risks, however. In Korea, the hostage situation has provoked a backlash. Bloggers and local media outlets have attacked the hostages for being naïve, and churches for competing with one another to see who can perform the most dangerous missions. Some Web postings even suggested that the hostages had gotten their just deserts.

Within the Christian establishment, the incident has triggered a reassessment. "Vacation missionaries [go] to war zones like Iraq and Afghanistan, and you get them in situations where they are way out of their depth," said Tim Peters, a Christian living in Korea. In the wake of the kidnappings, several churches and organizations have canceled their trips to Afghanistan. The Korean government has restricted its citizens from traveling to Afghanistan without explicit government approval.

Meanwhile, family members of the victims are gathered at Saemmul Church, praying and watching newscasts. Christians around the country are keeping vigil. Amid the onslaught of critical voices, many in Korea's Christian community feel misunderstood. "It's not about competition. I think missionaries are sharing because they have boldness," says Kim Hee-chan, who works at the Middle East Team, a group that helps organize missionaries. And, she says, "Missionaries sacrifice." A fact the hostages in Afghanistan know only too well.

Ms. Hook is an editorial writer at The Wall Street Journal Asia.