There’s nothing secret about Chongyi Church, one of the largest in China. Its lighted steeple and giant cross penetrate the night sky of Hangzhou, the capital of coastal Zhejiang Province.

Nearly everything at the church is conspicuously open: the front gate, the front door, the sanctuary, the people, the clergy. Chinese or not, you are welcome seven days a week. No layers of security guards or police exist. Walk right in. Join up. People are nice; they give you water, chat. Do you have spiritual needs? Visit their offices, 9 to 5.

For China, it is a stunning feeling. Most of the society exists behind closed doors and is tough, driven, material, hierarchical. The country values wealth, power, and secrecy – not to mention that both government and schools officially, at least, promote atheism.

Yet Chongyi looks and feels like any evangelical megachurch in Seattle or San Jose. There are big screens, speakers blaring upbeat music, coffee bars. The choir is a huge swaying wash of white and red robes. Chongyi seats 5,000 people and holds multiple services on Sunday.

“Some Sundays we are full,” says Zhou Lianmei, the pastor’s wife. “We also have 1,600 volunteers.”

While Christianity is waning in many parts of the world, in China it is growing rapidly – despite state strictures. The rise in evangelical Protestantism in particular, driven both by people’s spiritual yearnings and individual human needs in a collective society, is taking place in nearly every part of the nation.

Western visitors used to seeing empty sanctuaries in the United States or Europe can be dumbfounded by the Sunday gatherings held in convention center-size buildings where people line up for blocks to get in – one service after another. In Wenzhou, not far from Hangzhou, an estimated 1.2 million Protestants now exist in a city of 9 million people alone. (It is called “China’s Jerusalem.”) By one estimate, China will become the world’s largest Christian nation, at its current rate of growth, by 2030.

Indeed, an acute problem facing urban churches in China is a lack of space. Chongyi Church is building a million-dollar underground parking lot to replace one that worshipers under age 30 have taken over as a meeting place.

“I come because I found a love here that isn’t dependent on a person,” says Du Wang, a young businesswoman in Hangzhou. “It is like a river that doesn’t go away.”

Yet there is also trouble brewing for China’s faithful. As evangelical Christianity grows sharply, officials fear it could undermine their authority. Already, Christians may outnumber members of the Communist Party. That has far-reaching implications both for Chinese society and for a party that frowns on unofficial gatherings and other viewpoints. In China, party members cannot be Christian.

More than half of China’s Protestants attend illegal “house churches” that meet privately. The rest go to one of China’s official, registered Protestant churches, such as Chongyi. The official or legal churches, known since 1949 as the “Three-Self Patriotic Church,” operate under an arrangement that says in effect: We are patriotic, good citizens. We love China. We aren’t dissidents. We go to official theology schools. So the party will let us worship freely.

And – until recently – it has.

Yet in the past year authorities have attacked and even destroyed official Protestant churches, as well as unofficial ones. Many Evangelicals feel they are now on the front lines of an invisible battle over faith in the world’s most populous nation, and facing a campaign by the party-state to delegitimize them. Underneath it all is a question: Will China become a new fount of Christianity in the world, or the site of a growing clash between the party and the pulpit?

“There’s an enormous struggle across China brought by the rise of worshipers that seem to really believe,” says Terence Halliday, a director of the Center for Law and Globalization in Chicago who has worked in China. “Christianity now makes up the largest single civil society grouping in China. The party sees that.”

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In this photo taken July 16, 2014, church members pray in a private room in Jiu'en Tang, a Christian church, in the Shuitou township in Wenzhou in eastern China's Zhejiang province. AP Photo/Didi Tang When China opened and rejoined the world in 1979, US President Jimmy Carter asked China’s Deng Xiaoping for three “favors.” Mr. Carter asked that churches shut during the brutal Cultural Revolution be reopened. He asked that the printing of Bibles resume. And he asked that missionaries be allowed back into China. Mr. Deng accepted the first two requests, for open churches and Bibles. But he rejected the one for missionaries.

Thus began a slow restoration process harking back more than a century. The first Protestant church in China was built in 1848 in Xiamen, known then as the Port of Amoy. By the 20th century, American and British missionaries saw China as a rich field. Every city of importance had a church. Missionaries founded China’s first 16 colleges, and they spurred the first reforms for female emancipation.

But after Mao Zedong’s victory in 1949, authorities chased out the missionaries. During the Cultural Revolution from 1966 to 1972, officials closed and trashed churches as China turned violently inward. Mao partly justified the violence as necessary to bring China into the 20th century. But much of it was used to kill off his enemies, real or imagined, including the faithful.

The era produced “the most thorough destruction” of religion possibly in “human history,” write scholars David Palmer and Vincent Goossaert. Authorities threw Christians in prison. They burned Bibles and executed believers to make an example.

Philip Wickeri, a leading Anglican in Hong Kong, shows visitors two Bibles that illustrate how far things went in the 1960s, and how much they have changed since. One is a small plain New Testament made of mimeographed sheets embossed with hand-written Chinese characters. It is a Cultural Revolution-era “samizdat” Bible, painstakingly produced. Different church cells memorized single books of the New Testament, copied them, and then combined them to form a single gospel. The shadowy venture lasted several years, during which 150 Bibles were made.

Mr. Wickeri’s second Bible is gilt-edged and nestled in a rich box of bamboo. It is dated 2012 and was produced by the Amity Printing Company in Nanjing. It was part of a run that included the 100 millionth Bible published in China since the opening in the early 1980s.

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Children dressed as angels take part in a Christmas Eve mass at the South Cathedral official Catholic church in Beijing, China, Wednesday, Dec. 24, 2014. AP Photo/Ng Han Guan For decades, Christianity here was considered something for older female peasants. But the demographics of religion are changing dramatically. China’s new faithful are younger, more educated, more urban, and more affluent.

One surprising change is that a majority of believers no longer view Christianity as something foreign. They increasingly view faith as transcending its Western missionary-derived system. Many Chinese no longer accept the idea that being Christian means forfeiting a Chinese identity.

Last summer, China’s religious affairs chief said that 500,000 Christians are baptized each year in the country. A joint study between Baylor University in Waco, Texas, and Peking University in Beijing estimated that there are now 70 million Christians over age 16 in China. Communist Party membership is about 83 million.

Even so, no precise numbers exist for the total number of worshipers. Chinese government statistics put the rise in Protestants in the official churches at 800,000 in 1979, 3 million in 1982, 10 million in 1995, and 15 million in 1999. There the accounting stops.

Carsten Vala, an expert on religion in China at Loyola University Maryland in Baltimore, says 40 million to 60 million is “the low end of a conservative” estimate of the number of Evangelicals. Fenggang Yang, director of the Center on Religion and Chinese Society at Purdue University in Indiana, says he thinks there are more than 80 million Christians and that China will have 245 million by 2030 if growth is steady – making it the world’s most populous Protestant nation.

In some ways this surge seems counterintuitive. Being a Christian in a country that sees worship as odd or superstitious does nothing to boost one’s status. “There is absolutely no social advantage to being a Christian in China,” says Bob Fu, a pastor who escaped a Chinese police crackdown in the 1990s and now runs Texas-based ChinaAid, which monitors Christian rights in the country. “There are no cookies, no status, no outward rewards, no privileges in choosing Christianity.”

Yet as Chinese achieve material wealth and success, many feel lost. The success of economic reforms under Chinese leader Deng, launched in the early 1990s, has not helped rebuild China’s spiritual infrastructure, decimated during war and the Cultural Revolution. China’s rise has come with a cost: a loss of traditional values and the rise of cheating, corruption, and fierce competition. As Orville Schell, the Arthur Ross Director of the Center on US-China Relations at the Asia Society in New York, points out, there are 150 billionaires in China but little certainty.

“Everyone is groping and grasping,” he says. “People are turning to Buddhism, Christianity, self-help, and Taoism. CEOs and billionaires run around with their spiritual masters and visit meditation rooms.”

In dozens of interviews with believers in official and house churches, the word they use most for why they turn to church is “love.” “Chinese have a yearning heart, that is really the reason,” says one woman who goes to the Zion house church in Beijing, which has more than 10,000 attendees and whose pastor is Korean. “We need love, and in some ways it is that simple.”

One Chinese intellectual and former newspaper editor agrees that China has become sated and corrupt. But he doesn’t agree there is a significant turn toward spiritual matters.

“We are too comfortable and willing ... to say ‘yes’ to anything,” says Li Datong. “I wish there was more spiritual hunger.”

Yet Chinese parents complain of a society that teaches math and science in schools but does little to address conduct or character. The case of Little Yueyue is a symbol of the moral void. The 2-year-old girl was hit by a van in Guangdong a few years ago. The driver didn’t stop. The girl was thrown to the side of the road, and 17 people walked past before an itinerant migrant stopped to help. The event was captured on a video that went viral and spurred some national soul-searching.

Experts say the Chinese have a practical nature, and if they adopt the evangelical message, especially after years of required wrestling with Marxist thinking, they usually don’t take it lightly. Many work hard at it.

“Chinese Christians know the Bible better than some Southern Baptists,” says Wickeri in Hong Kong. “That’s not a small thing.”

Typical is the pastor Han Yufang at Chongwenmen Church in Beijing. Ms. Han is one of many women now being ordained in official churches. But for years her father forbade her to look into Christianity. She did anyway, studying it for seven years, the final two praying for most of each night. One evening she was on her knees by the bed and prayed to God, “Father, not my will but thine be done.” She says she felt a clear urge to study at a divinity school.

Another woman, a mother in her 40s, first went to church with friends. She says she felt nothing but kept going to be part of the group. She dabbled. She tried Buddhism, but, “for all the quiet, I never really found peace.” During one service the concept of “forgiveness came from nowhere and washed and melted me in a way I can’t describe,” she says. At the time she was “always fighting” with her husband. After the experience, the tension stopped. He also started attending church services with her, as did their son, who finds Bible stories “compelling.”

For the most part, Protestants try to keep the altruistic activities they do in society quiet and low-key. China officially recognizes five faiths – Islam, Catholicism, Protestantism, Buddhism, and Taoism. But only Buddhism and Protestantism are experiencing lively growth. Evangelicals do not want to draw attention to themselves and perform most of their good works without publicity.

Yet in cases such as the Sichuan earthquake in 2008, which killed 70,000 people, churches sent groups to help on the ground. By some estimates, as many as half the volunteers were evangelical.

Some Christians are trying to improve business practices and fight corruption as well. One business group asks members to pledge a “Ten Commandments” of good behavior that includes no bribing, no taking mistresses, no avoiding taxes, and no mistreating employees. Zhao Xiao, a researcher at the University of Science and Technology in Beijing, tells of a Christian in Harbin who lost $8 million his first year applying the principles but is now a leader in his industry.

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A child peers out near a cross on a gate of the Wangfujing Catholic Church in Beijing, China, Thursday, Aug. 14, 2014. Pope Francis' greeting to Chinese President Xi Jinping as he flew to South Korea early Thursday was a rare cordial exchange between the sides that have no diplomatic relations and are embroiled in a sometimes bitter contest for authority. AP Photo/Ng Han Guan One January morning last year in Hangzhou, Chinese officials showed up unexpectedly at the Gulou Church. It is a massive gray-stone edifice across the famed West Lake from the Chongyi Church. The Gulou clergy was informed that the cross on their edifice was scheduled to come down.

Church leaders were stunned. It was the first they’d heard of any plan to remove the cross. Then for much of the spring, they and other Christians in China heard of little else, as both official and unofficial churches were raided, destroyed, or dismantled in a campaign that has lasted more than a year.

Gulou itself was established by Presbyterian missionaries in the 1880s. The cross atop the steeple was enormous, a fixture next to a well-known highway overpass. It is dear to members as a symbol of their faith, says a pastor who declined to be named. For months, Gulou’s leaders delayed the removal of the cross. Meanwhile, authorities attacked churches and, as of this writing, have stripped or desecrated more than 426 of them, including knocking one down while President Obama was visiting Beijing last fall. In many cases, tearful worshipers surrounded the churches and scuffled with police. Zhejiang itself has become ground zero in China’s growing clash between church and state.

On Aug. 7 at 5 p.m., authorities returned to Gulou. They summoned the head pastor and said that at 10 p.m. the cross would be removed by crane. Word got out (the pastor only told one person since he could otherwise be jailed for calling an unofficial gathering). The church was surrounded by worshipers praying and chanting “cross, cross, cross.”

“We felt helpless,” a junior pastor says. “We told them how important this cross is, but they didn’t listen.”

“They can take the cross from our church,” he adds, “but they can’t take it from our hearts.”

Crackdowns on Christians are nothing new in China. What is different is how broad and systematic the suppression has been and how the state, for the first time, is attacking official churches. To be sure, it was clear by summer that Chinese President Xi Jinping was conducting a harsh roll-up of civil society in general – artists, lawyers, scholars, as well as Christians – as part of a new emphasis on orthodox party thinking and rules.

“The party isn’t satisfied with just keeping people behind a great firewall,” says one lawyer. “They actually want to indoctrinate.”

So far, the cross on Chongyi Church remains intact. But Evangelicals here who thought they were adhering to the proper political decorum are not happy. “People are angry and feeling betrayed,” says a local volunteer who did not want to be named for fear of retribution. “If I were the government I would not do this.”

Why authorities would alienate believers who think of themselves as loyal Chinese is unclear. Many local Christians first thought it was a mistake or something engineered by local authorities in Zhejiang Province. Officials said large crosses near highways were a driving hazard.

But as more churches lost their crosses, many far from highways, and other official churches were bulldozed, feelings changed. One church quietly offered to pay a series of fines, thinking the attacks were about money. “We were fooled at first,” says one local pastor. “Then we discovered they didn’t care about fines. They went after our crosses and gave the impression they enjoyed it.” The aim was to humiliate and shame, he says.

In recent years, Evangelicals in east China were “doing well,” the pastor continues. “But that is now changing. We are going backwards now. Everything is changing with the new leadership in Beijing. We know what is happening. We are not visitors here.”

Zan Aizong, a local journalist who became an Evangelical, says the government is trying to clamp down on churches and faith without causing a global outcry. Officials “use the legal system,” he says. “They go after crosses and building codes because it will not cause an uproar abroad. They want to turn Christianity into Chinese Christianity, controlled by the party.”

In August, amid the suppression in Zhejiang, the party issued a statement that it would soon unveil an official Christian theology. Wang Zuoan, head of China’s religious affairs ministry, told the state-run Xinhua news agency that Christianity was spreading so rapidly that a new theology was needed to avoid problems. “The construction of Chinese Christian theology should adapt to China’s national condition and integrate with Chinese culture,” he said.

As the attacks continue, church leaders are debating how to respond – whether to publicly challenge the crackdown or try to ride it out, the argument being that authorities could do much worse things if provoked.

“Many Christians are scared of the government,” says Ling Cangzhou, a Christian blogger in Beijing. “In China you rely on the government for jobs, position, for money. Families and relatives are affected. Dissidents don’t get promotion or advancement.”

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One effect of the new religious persecution in China is that it is bringing the official and unofficial wings of the Protestant Church closer. For years, the two sides have often been clashing siblings: In essence, private house churchgoers saw the Three-Self churches as compromised by the party. Official churches often saw house churches as misbehaving cults.

Yet now, as they share a common threat and as more young people take up Christianity who have little knowledge of the historical divide, the two wings are starting to converge, reinforcing a grass-roots movement that has already been under way for some time.

Worshipers are being introduced to Christianity in official churches and then moving to house churches for a deeper experience of Bible study and preaching. In turn, house churches are becoming less secretive and are reaching out to influence the official churches. “There is a growing but quiet cooperation among Three-Self pastors who aren’t as invested in the institution – who care more about church and the basic evangelical mission,” Mr. Vala says.

To be sure, real differences remain between the two sides. Three-Self pastors are trained at theology schools watched by the party. Mr. Zan, for example, attended one and says that former President Hu Jintao’s concept of a “harmonious society” was taught as something to emphasize in preaching, which Zan calls “propaganda.” “Official churches are not allowed to touch subjects like the Apocalypse or eschatology,” he says. “A lot of the preaching is about how to be good and loving and ethical, which is fine. But they are often antiseptic and less radical.”

Many house meetings last all day, whereas official churches have 60- to 90-minute services. “The [Three-Selfs] are too big,” says a musician from Anhui who started at an official church but moved on. “You can get lost in them. Smaller is more like home, more like the love you feel at home.”

In Beijing, the official Chongwenmen Church is near the train station, found by walking through a rabbit warren of streets and noodle shops. It is old and slightly creaky. Services are packed and believers are devout. Across town, the official Haidian Church is a huge white modernist structure in a high-tech zone. Outside there is a band and chorus and kids with “I [heart] Jesus” caps. People wait in line for services by the hundreds.

One private Calvary church feels much different. Set in a seminar room in an office tower, it seems far less institutional but more intimate. The pastor is from Taiwan and won’t talk with reporters. Yet in all three churches the focus is on Christianity as a life practice and not a philosophy, and of the Bible as a revelation whose meaning brings change and redemption.

During services at these churches in August, as the cross removal campaign intensified, pastors spoke openly of the “meaning of the cross.” Hymns sung included “ ‘Onward, Christian Soldiers’ ... with the cross of Jesus, going on before.”