A Discourse of the Religion Anciently Professed by the Irish and British

A landmark of Reformed scholarship, this 1631 treatise by Ireland's Archbishop of Armagh argues that the ancient Irish and British churches held doctrines indistinguishable from Protestant orthodoxy. Drawing on patristic sources, Celtic manuscripts, and early synodal records, Ussher examines Scripture authority, predestination, justification by faith, worship, sacraments, monastic life, and papal jurisdiction — demonstrating that Rome's claims over these islands rest on later innovation, not ancient tradition.