Diarmaid Ninian John MacCulloch is an English academic and historian, specialising in ecclesiastical history and the history of Christianity. Since 1995, he has been a fellow of St Cross College, Oxford; he was formerly the senior tutor. Since 1997, he has been Professor of the History of the Church at the University of Oxford.

St Cross College, known colloquially as StX, is a constituent college of the University of Oxford in England. Founded in 1965, St Cross is an all-graduate college with gothic and traditional-style buildings on a central site in St Giles', just south of Pusey Street. It aims to match the structure, life and support of undergraduate colleges, with the relaxed atmosphere of an all-graduate college.