During the twenty second year of Qian Long's reign, he passed the Closed Door Policy to limit the spread of Christianity in China. Guangzhou became the only trading port in China. Merchants are not allowed to land on China soil, but could only trade or stay shortly at sea port under supervision. Other than the 10 missionaries who stayed in the palace with positions as historians or astronomers, there were only limited missionaries who took care of the 300,000 believers in China. This situation persisted for 20 years.