The Elizabethan reign has lately emerged as a formative period for English ideas about the liberties of the subject and the ‘ancient constitution’ of the realm. Recent work has described the development of such ideas as having been driven by an organized campaign against the English ecclesiastical courts: a legal and intellectual effort that had emerged from the misgivings of certain ‘puritan’ lawyers about the powers claimed by new prerogative jurisdictions. The primary grievance of the campaign has been identified as having been the church courts’ use of the oath ex officio, and the campaign’s primary defensive tool has been identified as having been the twenty-ninth chapter of Magna Carta. But overlooked manuscripts reveal a more complex story. This article shows that the law of excommunication was as important to the campaign in question as Magna Carta. In addition, a re-examination of the life and work of James Morice, one of the principal lawyers responsible for the campaign, demonstrates that the law of excommunication deeply structured his understanding of the royal supremacy, and of the legal relationship between England’s secular and ecclesiastical polities – particularly as they had existed in the distant medieval past.