By 381, Christianity had become quite established. Internally, at the Councils of Nicea (325) and Constantinople (381), the church had settled its understanding of the Trinity. Externally, Christianity had become the religion of the Roman Empire. In 380, Emperor Theodosius I had decreed illegal all religions except that of “catholic Christians” (by which he meant mainstream Nicene Christians, i.e those that held to the faith proclaimed at the Councils of Nicea and Constantinople). At this time, there were even some cases of Christian religious violence, often spearheaded by monks, such as the destruction of the synagogue at Callinicum in 388, or the assault on the library in the pagan Serapeum temple at Alexandria in 391. This did not of course mean that paganism ceased to exist or was inactive, but it did demonstrate the extent to which, within 70 years, Christianity had come to dominate the Empire. The whole-church councils ("ecumenical councils") that met in the 300s and...