Bob Fu from US-based non-governmental organization China Aid said the rapid growth of people becoming Christians in China has caught the attention of the Chinese government.
Fu said: "We definitely have been seeing a major deterioration and worsening. That has to do with the overall President Xi regime's hardening policy."
Officially, the People's Republic of China is an atheist country but that is said to be changing.
Christian congregations in particular have skyrocketed since churches began reopening when Chairman Mao's death in 1976 signalled the end of the Cultural Revolution.
The Pew Forum on Religion and Public Life estimated over 67 million Christians in China.
The state only recognises the Protestant 'Three-Self Patriotic Movement', a government-aligned church based on the principles of "self-governance, self-support, and self-propagation."
Pastor Xu Yonghai holds services in an old apartment building in Beijing with a dozen Christians.
Speaking to the Special Broadcasting Service Corporation, he said: "We've been questioned by the police before. Friday is a less obvious day for us to meet."
While freedom of religion is guaranteed in China's constitution, many independent churches like Pastor Yonghai's are outside the Chinese Communist Party's control. Fu said that as a result, they're perceived as a "threat".
He added that churches previously allowed to grow without state interference have in recent years faced intense scrutiny, or have been destroyed.
However, Pastor Yonghai, who has been arrested three times for offences including 'smearing the government' in an article about the growth of Christianity, said the current crackdown is nothing compared to decades before.
He explained: "There hasn't been a day when a Christian in China wasn't in jail. But Christians before us in the '50s were given lifelong sentences, They were in jail for 20 years.
"Compared to that, what we face is nothing, right? In some things you take two steps forward, five steps back. Right now we're just in the five steps back stage."
