The Macartney Mission to China in 1793 is no doubt an important event in the history of Sino-British relations. While most people are eager to know for sure if Lord Macartney has kowtowed to Emperor Qianlong, there is another issue that deserves serious attention: Did the embassy ever request for permission for the Westerners to preach Christianity freely in China? In his second letter to King George III, Qianlong denounced the ambassador for requesting free preaching of Christianity in China. But Lord Macartney denied that he had ever made such a request. By consulting the archival materials from the East India Office, Archivium Propaganda Fide, and Università degli Studi di Napoli "L'Orientale," the present paper explains how the controversy has come about. Special attention is paid to the role played by the embassy's interpreter, Jacobus Ly, a Chinese missionary trained in Naples.