In 1690, the previously sympathetic Nguyễn ruler of Cochinchina (located in south-central modern Vietnam) prohibited Christian religious practice in his state. Uniquely in the history of Catholicism in early modern Vietnam, however, the ban did not lead to a persecution of believers. The following article, based extensively on archival materials from the Missions-Étrangères of Paris, historicises this event and the steps leading up to it in 1688–89. It argues that to understand what was happening on the ground in Cochinchina, and why, we need to analyse the way global and regional factors intersected with local, and even personal, ones to cause a prohibition of Christian practice in early 1690, an event for which internal Catholic dissention was almost entirely responsible.