South Korea is dealing with the largest daily increase of coronavirus cases in five months, with 279 cases reported on Sunday.
Many people are connected to the Sarang Jeil Church, where the pastor was a vocal critic of President Moon Jae-in.
Another church, Jesus' Shinchonji Church, was identified earlier this year as South Korea's largest viral group.
Conflicts were found to be linked to more than 5,200 cases.
What do we know about the current outbreak?
South Korea pg wallet
reported 279 new cases of the virus on Sunday, the first time since March it had more than 200 new cases.
Making the country's total number of cases at 15,318
At least 193 new cases have been linked to Sarangjel Church, according to the Seoul Metropolitan Government.
President Moon warned the number could rise as health officials rush to test all 4,066 church members.
He said the outbreak was the biggest challenge in the fight against the virus due to the group linked to the Shinchonji Church.
Lee Man Hee, leader of Shinchonji Church, was arrested earlier this month.
He was accused of hiding information about members of the group and the assembly from contact followers.
The church said he was concerned about the safety of its members. But never hide the information
What do we know about the Sarang Jeil Church?
Little is known about the church in Seoul - although more is known about Pastor Rejun Kwanghun, the leader.
The 63-year-old has been an outspoken government critic for years and is reported to have led several anti-government rallies in Seoul.
Over the weekend, he broke the separation rules by attending a rally by himself.
President Moon called on the church members who followed Mr Chun in the assembly, saying they took part in "Unforgiving acts that threaten people's lives"
According to the Korean Herald, Jun told his followers at the rally earlier this year: "Patriots who die of sickness", adding that "those who are sick will be healed if they join the rally".
Mr Jun was charged with defamation earlier this year after he called President Moon a North Korean spy, Yonhap reported.
On Sunday, the Seoul Metropolitan Government said it would take action against Mr Chun on charges of breaching self-quarantine rules and hindering officials' efforts to limit the spread of the virus.
South Korea currently limits indoor gatherings to 50 and outdoor gatherings up to 100.
It has been touted as a success story in dealing with COVID-19 after hitting low numbers earlier this year.
It has successfully used proactive monitoring and extensive testing to prevent the first outbreak. But the outbreak has been ongoing in recent weeks.