Jesus is a threat to all authoritarian regimes.
Some years later a bus of naive Korean Christians driving through known hostile territory was captured by the Taliban. The pastors were killed but the others eventually released. Subsequently the South Korean government refused to allow their citizens to go to Afghanistan. This was quite a blow to the overall international Christian community as the Koreans were spiritually strong, dedicated, and significant in numbers. Their participation in prayer gatherings and other events was sorely missed.
The problem is not religion, it is unlawful assembly. That has always been the concern of the Chinese government when dealing with what it considers underground churches. It can't see what is being organized when church service in not in an authorized location. Church bells ring calling people to worship service at the State sanctioned churches every Sunday and lots of people attend.