From the vantage point of Constantinople in AD 440, under the rule of the orthodox Theodosian dynasty, Nicene Christianity may have appeared unassailable. Many of the most challenging of the heresies that had emerged in the century since Constantine’s conversion seemed to be receding. Even the devastating Christological controversies that had erupted in the early 430s may (understandably, if wrongly) have looked, with Nestorius’s deposition and the Formula of Reunion (433), to be under control. And yet Constantinople retained its confessional diversity, with members of numerous Christian sects – penalized by law theoretically, but often still visible and vocal in practice – living alongside members of the mainstream Nicene church.
The reimposition of a Nicene hierarchy over the major churches of Constantinople had happened only from the 380s onwards. It had been a process fraught with difficulties – not least the survival in large numbers of groups polemically labelled as Arian, and the persistence of their intellectual, theological and historical traditions, even as their hopes of political or theological hegemony faded. From this milieu had come the Eunomian Philostorgius (c.368–439), and his highly sectarian – and anti-Nicene – Church History of the fourth and fifth centuries.