Chinese officials are aware of the underground work of Korean Christians, but have had to accept Deng Xiaoping's axiom that sometimes when you open the window, flies come in.
But the missionaries will be harder to ignore after the announcement this week that an American of South Korean descent had been charged with spying after crossing into North Korea from the Yalu River border with China.
North Korea has charged that Evan Carl Hunziker, 26, a native of Washington state, was conducting espionage for South Korea. Officials in Seoul deny the charge, saying Hunziker was a missionary based in China.
His father, Edwin Hunziker of Parkland, Wash., told the Associated Press that his son converted to Christianity last year and probably was ``only trying to preach the Gospel'' when he was arrested. He described his son as a drifter with alcohol and drug problems who converted while jailed in Alaska on a drunken-driving conviction.
The U.S. government also disputes the spy charges, calling them ``ridiculous.'' But it has released few other details about Hunziker, who is being detained in a Pyongyang hotel.
U.S. officials have said the charges against Hunziker, whose mother was born in South Korea, are retaliation for international condemnation of North Korea after North Korean commandos landed in South Korea in a spy submarine last month.
``It would be outrageous and indefensible should the North Koreans try to link the submarine incident with this unfortunate young man,'' State Department spokesman Nicholas Burns said Tuesday.
If Hunziker is a missionary, as the South Korean Foreign Ministry asserts, his exploits have brought unwanted attention to the work of foreign church groups in China.
Foreign missionaries are permitted to preach in China only if the government invites them, and that is a rarity. Existing churches - most of which are holdovers from before the Communists' rise to power in 1949 - are kept under tight control. Chinese are allowed to practice religion, but the converting of Chinese citizens, even by fellow Chinese, is prohibited.
Many Korean missionaries get around the ban by coming to China as teachers. Beijing knows this. Last summer, the Chinese government issued a stern warning to Seoul that it would clamp down on teaching visas in order to stop missionary work.
The lure to China for Korean missionaries is strong not only because so many ethnic Korean believers live along the border, but also because they have their eye on North Korea as well.
Korean church groups from the South are waiting for the day when the peninsula is unified. In the meantime, they are working secretly to do as much as they can from China.
``Northeast China is seen as a beachhead for proselytizing in North Korea, which was the cradle of Christianity on the Korean peninsula before World War II,'' said a Western diplomat here. ``It's the perfect launching pad.''
The border between China and North Korea is fluid. Ethnic Koreans who live on the Chinese side are free to travel in and out of North Korea.
With the help of Korean Christians, not only from South Korea but the United States, Canada and Australia as well, some have been able to sneak Bibles into the country.
``One Korean American Christian told me that Pyongyang is our Jerusalem,'' the diplomat said.
More than any other place in Asia, Korea was fertile ground for Christian missionaries from Europe and America at the turn of the century.
Christianity took hold because it was linked to strong feelings of Korean nationalism. Even though Protestant and Catholic religions were the products of foreigners, Christianity was viewed as a way to assert a Korean identity against Japanese rulers, who annexed the peninsula in 1910. During the Japanese occupation, Koreans were forced to worship the emperor, bowing to him every morning, and to follow Shinto beliefs. Christianity was considered a protest against that.
Before World War II, large numbers of Christians lived in what is now North Korea, with Pyongyang a center of religious activity. The mother of Kim Il Sung, the former cult communist leader, was a Christian convert. But after Kim swept to power after the war, his communist regime tried to purge the North of Christians, executing believers and clergy.
Today, the only way for Koreans from the South to reach Christians still living in the North is through China.
Such contact, although indirect, became possible only four years ago, after China and South Korea renewed diplomatic relations. It took the countries so long to restore relations because China is one of the few allies of North Korea, the avowed enemy of the Seoul government. But for China, the lure of possible South Korean investment became too great to ignore.
Today, charter flights fly directly from Seoul to provinces in northeast China, packed with South Korean tourists and businessmen. An official for the South Korean Embassy here said 500,000 South Koreans came to China last year, with the number expected to increase to 700,000 this year.
In the small city of Yanji, less than 75 miles from the North Korean border in Jilin province, the flow of South Korean money is plain to see. In the heart of the city, the Daewoo conglomerate has built a new five-star hotel. A few miles away, a small, one-story Protestant church is moving into a new, grander building: a towering brick church that will seat 3,000.
Sticking with the party line, a representative for the church said the money to build the six-story edifice was raised locally. But some observers think the funds for such an imposing cathedral could come only from one source: South Korea.