The purpose of this tripartite paper is to demonstrate the unique and exceptional value of the Epistola ad milites Corotici as a direct source for the historical reconstruction of the origins of the Celtic insular Church in the early years of so-called Sub-Roman Britain. Patrick’s letter, as I will try to show, contains information which is not otherwise available on these crucial years. Moreover —and on occasions in comparison with the Confessio—the value of the Epistola in relation to these events has increased. The themes and historical figures discussed in the Epistola have proved to be fundamental to an understanding of the historical, social and religious causes both of the end of Roman power in Britain and the sudden rise of the apostolic missions especially in Ireland and in the rest of the Post-Roman Britain, as well as in less Romanised areas as a result of Nynias’s and Columba’s preaching and by means of the definitive action of the Roman Church in sending Augustine of Canterbury to Britain.