Turning from the disagreements between pagans and Christians over the causes of the sack of Rome to disagreements between the Christians themselves—Gibbon's “sects of Christianity”—no one could deny the fact that the fourth, fifth, and sixth centuries were riven by doctrinal conflict. This was a key period in the definition of orthodoxy and, as a corollary, the designation of doctrinal opponents as heretics. Without considering any of the minor heresies one can list Arianism, Donatism, Priscillianism, Pelagianism, Nestorianism, and Monophysitism as doctrinal positions that caused major conflict in the course of the three centuries following the conversion of Constantine. Some, though not all, modern histories of the fall of Rome have integrated these doctrinal disputes into their narratives.

Ecclesiastical historians, of course, have most certainly not lost sight of the importance of doctrine, or of the sig-nificance of the ecumenical councils, which have attracted notable attention in recent years. The wider social and political implications of doctrinal debate, however, were rather more central to scholarship in the 1970s than is now the case. In 1972 Bill Frend published his account of The Rise of the Monophysite Movement, in which he placed the religious debates over the nature of Christ in the fifth and sixth centuries firmly at the centre of his reading of the history of the early Byzantine World. But he had already looked at the history of the African Donatist Church in a similar vein in 1952. Frend's arguments led to heated discussion as to whether ancient heresies were national or social movements in disguise. Most scholars would see the debate as long concluded, and as it was formulated by Frend this is surely the case. Yet equally there is no doubt that the debates over Monophysitism in the fifth and sixth centuries caused lasting divisions in the eastern Empire—divisions which Justinian's attempts to solve did nothing but exacerbate. Indeed, in condemning the writings of Sozomen, Socrates, and Ibas in an attempt to win over some of the critics of the settlement at Chalcedon, the emperor caused the controversy of the Three Chapters, which for a while split East and West, and continued to trouble the West into the seventh century.