Ussher certainly preferred to be a scholar when he could be. He engaged in extensive disputations with Roman Catholic theologians, and even as a student he challenged a Jesuit relative, Henry Fitzsimon (Ussher's mother was Catholic), to dispute publicly the identification of the Pope with the Antichrist. Ussher had an obsession with "Jesuits disguised as" Covenanters in Scotland, highwaymen when he was robbed, non-conformists in England, it was a remarkable list. However, Ussher also wrote extensively on theology, patristics and ecclesiastical history, and these subjects gradually displaced his anti-Catholic work. After Convocation in 1634, Ussher left Dublin for his episcopal residence at Drogheda, where he concentrated on his archdiocese and his research. In 1631, he produced a new edition of a work first published in 1622, his "Discourse on the Religion Anciently Professed by the Irish", a ground-breaking study of the early Irish church, which sought to demonstrate how it differed from Rome and was, instead, much closer to the later Protestant church. This was to prove highly influential, establishing the idea that the Church of Ireland was the true successor of the early Celtic church.