Three years after a few hundred brave souls celebrated their first midnight mass in a decade, approximately 5,000 Catholics crammed into St. Mary's tonight to sing "Silent Night" in Chinese and pray in Latin.

But their sense of religious exhilaration is tinged with uncertainty.

Their concern stems from the peculiar and vulnerable role of the Catholic Church, which was allowed to operate even before the Cultural Revolution on the condition that it sever all relations with the Holy See in Rome. The Vatican has incurred the wrath of China's Communist officials by continuing to recognize Taiwan.

China's Catholics, who numbered 3 million to 4 million before the Communist takeover in 1949, belong today to what is called the Patriotic Church, which appoints it own bishops and shuns the Vatican.

Despite the ban on Roman Catholicism, underground sects still loyal to the pope reportedly meet. They monitor papal pronouncements by scanning foreign newspapers and listening to overseas radio stations.

Last month, four priests and several Catholic laymen were arrested in Shanghai on charges of maintaining ties to the Vatican and circulating rumors that the Virgin Mary would reappear at a church outside Shanghai.

A Communist official explained the arrests as necessary to stop "infiltration by reactionary foreign religious forces . . . who have followed the wishes of the Vatican and tried to undermine the independence of the Chinese churches."

For this official, Zhang Zhiyi, and other members of the breakaway Chinese church, the continued activism of Roman Catholics is seen as a two-pronged threat: it splits the Chinese church, and it could prompt Communist leaders to reconsider their tolerant stance.

To help ensure their church's survival, Chinese Catholic leaders protest the loudest whenever the pope's influence appears to be seeping into China. When the pope tried last June to name Chinese Bishop Dominic Tang as the Vatican's representative in Canton, the Patriotic Church called the appointment illegal and fired Tang from his Chinese positions.