This article examines the late-seventeenth-century Church of England's understanding of rulers’ ecclesiastical imperium through analysing a pamphlet debate about Julian the Apostate and Church-state relations in the fourth-century Roman empire. In 1682 an Anglican cleric, Samuel Johnson, printed an account of Julian's reign that argued that the primitive Christians had resisted the emperor's persecutory policies and that Johnson's contemporaries should adopt the same stance towards the Catholic heir presumptive, James, duke of York. Surveying the reaction to Johnson, this article probes the ability of Anglican royalists to map fourth-century Roman onto seventeenth-century English imperium, their assertions about how Christians should respond to an apostate monarch, and whether these authors fulfilled such claims when James came to the throne. It also considers their negotiation of the question of whether miracles existed in the fourth-century imperial Church. It concludes that, despite Rome's territorial dimensions, imperium remained a fundamentally legal-constitutional concept in this period, and that the debate over Julian highlights the fundamentally tense and ambivalent relationship between Church and empire.