The Sino–U.S. Treaty of Tientsin (1858), for the first time in history, included an article to ensure the toleration of Protestantism in China, i.e., the Toleration Article. The man behind the article was S. Wells Williams (1812–1884), an interpreter of the U.S. Legation and a former missionary, who negotiated with Chinese officials directly. Williams produced the Chinese version of the clause and its official English translation which led to controversies, for it was deemed unfaithful and humiliating to the Christian communities. This paper revisits this important episode in the history of Protestantism in China. Using first-hand archival materials, it reveals how Williams was forced to “mistranslate” the article in order to satisfy both parties. It further discusses the reasons behind the missionaries’ criticism, which placed Williams in the center of clashes between religious and political interests. Through Williams’ case, the paper foregrounds the crucial roles and difficult situations of missionary translators in diplomatic incidents that shaped the history of Christianity in China. More importantly, it highlights the conflicting interests and ideas in early Sino–U.S. diplomacy and shows how missionary translators worked to mediate the dilemma between evangelism and politics in 19th-century China.