SEOUL, South Korea — For months, the red-brick church in a rundown neighborhood of Seoul, the South Korean capital, has attracted thousands of politically active conservative Christians, all united in the belief that their country is falling into a godless communist hell ​under the leadership of its liberal president, Moon Jae-in.

Devotees of the church, known as the Sarang Jeil Church, whose name means “love comes first,” have participated in some of the largest antigovernment protests the country has seen in years.

“If we hesitate, it will not be long before we live under the ‘great leader’ of North Korea. Do you want that?” the Rev. Jun Kwang-hoon, the church’s chief pastor, said during a large antigovernment rally in central Seoul last Saturday.

Now their political crusade is colliding with the coronavirus, as a large outbreak centered on the church spreads fast through Seoul and beyond, threatening the country’s success in fighting the pandemic.

Mr. Moon has accused his most vocal critics of spreading the infectious disease and putting the entire nation in danger — a sentiment widespread on social media. Police officers have been sent to track down Sarang Jeil congregants who have broken quarantine.

But in today’s polarized South Korean society, fraught with fake news, conspiracy theories and fear-mongering, alternative narratives have also taken hold, purporting that the congregants have become the target of a political witch hunt or even a terrorist attack from communists.

Conservative activists have accused Mr. Moon of trying to scapegoat the church to divert attention from his weak approval ratings, which have been plummeting over domestic policy blunders like soaring housing prices. Church officials even suspect health officials manipulated virus-test results to keep Mr. Moon’s die-hard critics quarantined.

In the past week, the outbreak has forced the church to shut down, and its congregants to isolate themselves at home. The infections among church members and their contacts have spiked to 676 cases, including Mr. Jun.

The outbreak pushed South Korea’s daily caseload to 288 on Thursday, the seventh straight day of triple-digit jumps, which shattered hopes that the country had managed to blunt the epidemic sooner than most nations. It marked the biggest cluster of infections in South Korea since an outbreak in the Shincheonji Church of Jesus in the central city of Daegu in February and March was tied to 5,200 patients.

Health officials have warned that the outbreak at Sarang Jeil could prove far more devastating than Shincheonji’s.

It has erupted at the center of the Seoul metropolitan area, home to half the country’s 51 million people. Sarang Jeil’s congregation is much older and could prove weaker to the virus than that of Shincheonji.

Unlike Shincheonji’s secretive congregation, many of Sarang Jeil’s 4,000 congregants traveled from across the country to attend Mr. Jun’s sermons and political rallies in Seoul. Health officials have been racing to track them down for testing and isolating, warning of “massive nationwide transmission.”

“The number of people coming to our church has ballooned in recent months, although not all of them are registered as our members,” said Han Hwan-ho, 51, a worshiper at Sarang Jeil since 1987. “On ​weekends, there were so many traveling from other cities that the church quickly filled up and many had to sit outside in the alleys in plastic chairs.”

​Mr. Han said people flocked to the church “out of fear that our country is falling under communist influence, and to defend our country’s alliance with the United States and our freedom of religion.”

“​I am convinced that our pastor was speaking like a prophet when he said that our country was in danger of communization and that we would lose our religion when that happened,” he added.

South Korean politics have long been an ideological battleground.

Liberals have championed reconciliation with North Korea and favored a “balanced diplomacy” between the United States, South Korea’s most important military ally, and China, its biggest trading partner. Conservatives, especially older Christians, have loathed North Korea, feared China and regarded anything less than unequivocal support for the alliance with Washington as “communist.”

Conservatives lost power when South Korea impeached President Park Geun-hye, a right-wing icon, on corruption charges, replacing her with Mr. Moon, a liberal, in 2017. Mr. Moon’s Democratic Party won in a landslide in parliamentary elections in April, thanks to his government’s successful fight against the coronavirus.

Older conservatives deeply mistrust Mr. Moon, accusing him of putting South Korea under the influence of North Korea and China at the expense of its alliance with the Americans. But they feel voiceless, as the conservative political opposition remains unpopular and disarrayed in the aftermath of Ms. Park’s impeachment.

Ultra-right-wing Christian activists moved from the political margins to fill the vacuum. The star among them was the Mr. Jun, the main architect of a faith-based, conservative political activism in the country.

At his political rallies, people waving South Korean and American flags have prayed to God to unseat Mr. Moon and condemn North Korea’s leader, Kim Jong-un, to hell. They have shouted “Hallelujah” and spoken in tongues.

“They believe that Christianity cannot share the same house with communism,” said Hwang Kyong-gu, a popular YouTuber who leads a conservative activist group named the Korea Patriotism Patrol Team. “They see their campaign against Moon Jae-in as an ideological conflict: a free world versus communism.”

South Korean Protestant churches have deep ties with the United States. American missionaries brought the religion to Korea.

Many of the megachurches in South Korea were founded by Protestants who fled communist persecution in North Korea before the 1950-53 Korean War and benefited from postwar aid from American churches. To older Christian conservatives who remember the carnage of the war and the poverty that followed, religious faith remains synonymous with anti-communism and loyalty to the alliance with the United States, which defended South Korea during the war.

Mr. Jun has roused these old sentiments with sermons replete with expletives against Mr. Moon. He calls Mr. Moon a “chief North Korean spy” and urges his followers to become “martyrs” in a war to drag him and other “North Korea followers” out of the presidential Blue House.

“He speaks in a language​ his audience can understand​ and like to hear​,”​ said Hwang Gui-hag, the editor in chief of the Seoul-based Law Times, which specializes in church news.​ “He scratches them where it itches the most.”

Health officials are now investigating the source of the virus in the Sarang Jeil congregation. The first case was reported on Aug. 12.

Amid a surge in infections, the government ordered congregants to stay home last week. But on Saturday, at least 10 church members, including Mr. Jun, attended the anti-Moon rally in Seoul, health officials said.

Mr. Moon called their behavior “an unpardonable act against the safety of the people,” accusing them of impeding the government’s efforts to fight the disease. Sarang Jeil officials said they had enforced preventive measures against Covid-19 during their church gatherings, and were urging all members to cooperate with the government.

A deep antigovernment sentiment among church members could impede the health authorities’ efforts. Thousands of police officers were mobilized to track down more than 500 church members who remained unreachable although they needed testing.

This week, ​in ​the southern city of Pohang, a woman tested positive after attending Mr. Jun’s church gatherings. Before officials could take her into quarantine, she had run away with her Bible after biting her husband, who tried to stop her. ​She was later detained by police officers wearing full-body protective gear. Another participant in the pastor’s rally fled a government-run quarantine center and was hanging out in cafes in Seoul when the police nabbed him.

Kim Kyong-jae, a conservative activist who helped organize the Saturday protest, said Mr. Moon’s government was “witch-hunting” the church and ruling with “quarantine dictatorship.”

Mr. Han and the Rev. Lee Eun-jae, an aide to Mr. Jun, said that many church members suspect the government manipulated the test results to keep them quarantined, and said that some avoided free government-provided tests and instead got themselves diagnosed in private clinics. Health officials called such fears groundless.

Before he himself was hospitalized, Mr. Jun claimed that the outbreak at his church was a result of a “terrorist attack with the virus from Wuhan, China.”

“An invisible hand from the outside was involved,” he later said in an interview with a Christian website. “Broadly speaking, it could be an act perpetrated by North Korea.”