Rev. C. has nearly finished his latest book, a compilation of daily devotions for pastors in China. To get his manuscript from Hong Kong into the hands of his students on the Chinese mainland he’ll have to — well, for his safety that can’t be published. Neither can his name, since he agreed to speak to TIME on condition of anonymity. So let’s just say this slight and soft-spoken Protestant has spent years giving Chinese authorities the slip to deliver his spiritual message to Chinese Christians.

Rev. C. is convinced that Christianity alone can shake the Chinese Communist Party’s (CCP) indomitable grip. He’s willing to go jail for this conviction. In fact, he already has.

“It’s a blessing to go to prison,” he says, “to suffer for Jesus.”

He’s not alone. While Hong Kong’s pastors are not allowed to proselytize, sermonize or establish churches in mainland China without official permission, many defy these prohibitions to cultivate a network of underground “house churches” in homes and workplaces.

Hong Kong has historically served as the springboard for evangelizing on the mainland. But as President Xi Jinping kicks off a renewed crackdown to bring Christianity under state control by instituting new religious regulations, pastors in Hong Kong — since 1997 a semi-autonomous Chinese territory — are finding themselves in the crosshairs.

“The Communist Party of China is afraid of this thing. They want to control the Christians,” says Rev. C.

Christianity, he says, has grown too big in the eyes of Beijing, which has historic reason to fear the politicization of religion.

One hundred and sixty-eight years after Christian-inspired rebels nearly brought China’s Qing Dynasty to its knees in the Taiping Rebellion, communist China looks set to host the largest population of Christians in the world by 2030 — a development that is no small source of anxiety for the officially atheist country’s authoritarian leaders.

The Gateway Into China

Proselytizing may be forbidden on the mainland, but step off Hong Kong’s iconic Star Ferry and into the audio and visual assault of ticket touts, digital billboards, souvenir hawkers and street acrobats and you’ll find Christians come to spread the gospel. As selfie-stick wielding masses jostle in front of the city’s harbor and glass skyline, leaflets attesting to Jesus’ love and eternal redemption are pressed into the hands of mainland tourists.

Hong Kong, with its greater freedoms and religious liberties, has played a vital role in oxygenating the growth of Christianity on the mainland.

Unlike in many parts of the West where Christianity is waning, a religious gold rush has swept through China since the Cultural Revolution and its fierce suppression of religion ended in 1976. Scholars estimate there are now as many as 80 to 100 million Christians, compared to 89.5 million communist party members. As more and more Chinese seek a spiritual alternative to political repression, Christianity continues to gain ground, increasing by an estimated 10% per year.

While Christianity is undoubtedly thriving in mainland China, faith is permitted only in official, “patriotic” churches; unregistered houses of worship may be prolific, but they are also subject to periodic crackdowns. According to Christian advocacy group China Aid’s most recent statistics, 1,800 house church leaders were detained in 2016.

For these underground congregations — which are illegal, if often ignored — the Hong Kong Christian establishment offers a vital lifeline, supplying everything from monetary support, to Bibles, to blacklisted Christian literature, to training and assistance founding new churches. The gospel is smuggled over the border in every format imaginable: broadcast on pirate radio waves and disseminated through USB flash drives.

“They need our help because we are in the freer world and they are not,” says Hong Kong’s retired Catholic Cardinal Joseph Zen.

For evangelicals eager to sustain this fount of converts, Hong Kong serves as “the stepping stone into mainland China,” says Rev. Wu Chi-wai, general secretary of the Hong Kong Church Renewal Movement.

More than 60% of Hong Kong’s churches engage in work on the mainland, illicit or otherwise, including preaching and theological training, according to the Church Renewal Movement’s most recent, 2014 survey. They do so armed with Bibles, sermons, and, if the work is not officially sanctioned, an arsenal of disguises and convoluted transportation plans to counter omnipresent state surveillance.

Such business can be risky, resulting in anything from police harassment to deportation or detention in “re-education” centers. But as Rev. C. says, “Many church leaders believe that if you have not yet been to prison you are not committed enough in your faith.”

While China’s faithful have rapidly multiplied in number, they lack experienced leadership and qualified pastors. So Hong Kong has become a central hub for short-term theological intensives, distance Bible seminaries and networking conventions.

Read More: Risen Again: China’s Underground Churches

“Hong Kong’s role is to help them become a self-propagating, self-administrating establishment,” says another Hong Kong missionary, who, like Rev. C., could not be named for safety reasons.

But the future of this relationship is threatened by a revision of the 2005 religious regulations which came into force last month. The 77 vaguely worded provisions indicate the government’s priorities as it doubles down on extralegal worship amid a broader push to cement party-state authority.

For the first time, religious exchanges with Hong Kong, Taiwan and Macau have become a target. China’s house churches were previously barred from “foreign affiliations,” but now any religiously motivated trips abroad must be vetted by Beijing.

“According to the new regulations, believers from mainland China are forbidden to attend unauthorized overseas religious conferences or training, or serious penalties will be imposed. Hong Kong is part of the overseas areas,” says Bob Fu, president and founder of China Aid.

Many Hong Kong pastors are suspending or outright canceling their work for fear of endangering their followers.

“Now is a sensitive time. Many pastors tell me they will have to wait and see how [the regulations] are enforced,” says Rev. Wu.

A “Subversive Seabed”

Many pastors say Beijing’s interference in their work is symptomatic of China’s encroachment on Hong Kong’s political autonomy.

“Beijing sees Hong Kong as place of insurgency, a place that needs to be brought under control,” says Brynne Lawrence, an associate at China Aid.

From China’s perspective, Hong Kong needs to be reintegrated into the mainland, political economist and Hong Kong transition expert Michael DeGolyer writes in The Other Hong Kong Report, a Hong Kong-based academic journal. While Hong Kong enjoys greater liberties than the mainland under the “one country, two systems approach” instituted after the 1997 handover from British to Chinese sovereignty, DeGolyer describes this agreement as a temporary transition period during which differences generated during 150 years of separation are to be respected, and overcome.

Rev. Wu says Hong Kong has long been seen as the “subversive seabed” from which provocative ideas — religious or secular — seep into the tightly controlled mainland.

In 1923, nationalist revolutionary leader Sun Yat-sen defined Hong Kong as ground zero for resistance.

“Where and how did I get my revolutionary and modern ideas? I got my ideas from this very place, in the colony of Hong Kong,” said Sun, who attended the first independent Chinese church, founded in Hong Kong.

Members of a house church meet for Sunday service May 15, 2011 in Beijing, China. Jonathan Saruk—Getty Images

The enclave has long served as a harbor for agitators and insurrectionists. It was a hotbed of communists during the 1920s and ‘30s, a base for Japanese imperialism in the Second World War, a sanctuary for nationalists fleeing the PRC, a refuge for Russian émigrés fleeing the Bolshevik Revolution, a home in exile for Indonesia’s national hero and communist leader Tan Malaka, a source of funding, supplies, and ideological encouragement for the Tiananmen Square protesters, a safe haven for NSA whistle-blower Edward Snowden and, most recently, the birthplace of the pro-democracy Umbrella Movement. Beijing’s flag-waving state media did not fail to note that several Christian leaders helped spearhead those 12-week Occupy protests in 2014.

“[Nobody is] allowed to use Hong Kong for infiltration subversion activities against the mainland to damage its social and political stability,” Zhang Xiaoming, the head of China’s Liaison Office in Hong Kong, said during a state media interview last year.

The admonition appears to extend to Christian evangelizing.

“They do no want the water from the well poisoning the river,” says Cardinal Joseph Zen.

The Chinese Communist Party has long associated Christianity with subversive Western values, which are perceived as antithetical to Xi’s push for conformity to orthodox party thinking. Xi has even said the government “must guard against overseas infiltrations via religious means and prevent ideological infringements by extremists.” He advised religions to Sinocize by accepting Chinese traditions and socialist core values, which really means submitting to state authority.

Growing Crackdowns

Religious leaders say hostility toward Christianity peaked under Xi, who became party leader in 2012 and has presided over a crackdown on civil society to quash dissent and establish what academics have termed his complete “controlocracy”.

“They don’t want to totally restrict religion, they want to bring it fully under their control,” says the Hong Kong missionary.

Christian groups say sporadic persecution has intensified and campaigns to demolish unregistered churches, tear down crosses, raid homes for unauthorized gospel literature, arrest church leaders and monitor congregants have all become more common. Last November, local authorities in Jiangxi province reportedly told residents to take down Christian iconography inside their homes and replace it with portraits of Xi.

The sweeping new religious regulations “try to legitimate the repressive measures adopted in the past few years,” and provide a legal framework for future crackdowns, says Yang Fenggang, director of the Center on Religion and Chinese Society at Purdue University.

Aiming to curb unregistered religious activities, the regulations give underground churches an ultimatum: join the official, antiseptic Three-Self Patriotic churches where faith is subordinate to party dogma, or face criminal repercussions left to local enforcers’ interpretation — traditionally anything from fines, to detention or even enforced disappearances.

“In the U.S., the citizens could say that the law protects us, the first amendment protects our religious freedom. In China it’s the other way around. The law is just to help the government crackdown on the churches,” says Rev. Wu.

A believer prays during a weekend mass at an underground Catholic church in Tianjin in Nov. 10, 2013. Kim Kyung Hoon—Reuters

To cope in such a hostile environment, China’s underground churches have adopted guerilla-like tactics. Rev. C. described Christians who use balloons to obscure their faces from CCTV cameras while they walk to church, shops that act as fronts for Sunday schools, and coded conversations that allow pastors to talk openly about planting new churches.

“China’s Christians have endured decades of persecution,” Rev. C. says. “They know how to deal with the Chinese government.”

Plus, he adds, “Beijing can’t arrest them all. There are too many Christians now and not enough jails.”

It’s Hong Kong’s future, and the ability to adapt to unfamiliar oversight from Beijing that he worries about. “We’ve been safe here for the last 20 years. In the coming years? We just don’t know.”

Few religious leaders were optimistic in their forecast for the metropolis.

Read More: China’s Religious War: Cardinal Zen Talks Beijing and the Vatican

Cardinal Zen said those who believe in the perpetuity of Hong Kong’s sovereignty under the “two systems” approach are blind to its steady erosion. “Here we have no future unless we want to be Beijing’s slaves,” he put it bluntly.

One Christian academic, who asked not to be named, tells TIME that Hong Kong’s liberties — including free expression — are withering fast under the unfavorable attentions of Beijing.

“My worry is that some church leaders in Hong Kong are surrendering,” the academic says. “They just obey the government and do whatever they are told, keeping their mouth shut and not daring to criticize policies. You can already see this happening.”

Party vs. Pulpit

Trouble began brewing even before the rollout of the new regulations. Mainland Christians were sporadically barred from attending conferences and conventions in Hong Kong, and Hong Kong pastors have increasingly paid a price for trying to spread the gospel beyond the territory’s border.

In 2016, China Aid held a training in Hong Kong attended by over 400 mainland Christians. Not long after the event, Fu said three facilitators from the Chinese University of Hong Kong faced repercussions when they tried to visit the mainland: in some cases they were beaten, and in others warned.

“The authorities have their lists. If you are on the list, you have become a target, and you are not allowed to cross the border,” says Rev. Wu.

In an unprecedented incident portending the tightening restrictions to come, in 2015, Rev. Philip Woo was summoned from his Hong Kong office across the border. Religious affairs authorities there instructed him to stop teaching mainland students, and to stop posting online advertisements offering to ordain mainland pastors. Since then, he says he’s also been warned by Hong Kong’s authorities to call off trips to the mainland, where he has been unable to return for over a year.

“The Chinese government should not be trying to interfere,” he says.

But for the Communist Party, there are practical reasons to clamp down, says Fenggang, from Purdue University.

Christians, drawn to the faith’s moral compass, “have shown the will to challenge the injustice of the party-state,” he wrote by email. “Their presence is a challenge to the moral authority of the party-state. The more the party-state feels the lack of moral authority, the more it [will] try to suppress Christianity.”

Participants raise their hands in prayer during the first Global Chinese Alpha conference in Hong Kong, April 10, 2007. AFP/Getty Images

Yet paradoxically, the more severe the persecution, the more people are drawn to Christianity.

“By clamping down on it, the Communist Party has multiplied it,” says Carsten Vala, chair of the political science department at Loyola University.

He also noted that while most Chinese Christians are not interested in seizing political power, Christianity and communism are inherently at odds, competing over the souls and loyalties of the people.

“Protestants have arguably created the most sustained structural challenges to the Chinese Communist Party’s ordering of society,” Vala says.

Rev. C. says he is motivated by the belief that if Christianity continues to grow in China, it’s conceivable that 20-25% of the country could be Christian. At that point, he says, “the Communist Party will not be able to handle it.”

“With Christianity [there will be] morals, ethics, just laws, and a will to enforce it,” he says.”Only Christianity can change this country.”

With reporting by Aria Chen / Hong Kong.

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Write to Laignee Barron at Laignee.Barron@time.com.