The story of Western Christianities from Constantine to the close of the sixth century is one of both expansion and the formation of diverse Christianities. The expansion is slow and difficult to trace: at the beginning of the fourth century, the Western regions of the Roman empire were much less Christianised than the East, only an estimated 2 per cent of the population. Although the progress can be tentatively gauged from the archaeological and epigraphic records or from the multiplication of episcopal sees, a general picture is difficult to establish. The countryside presumably resisted Christianisation (if it ever became completely Christian) far longer than the urban population; missionary efforts by bishops or monks (if they occurred) changed little. The Christianisation of Western aristocracies, on the other hand, has been comparatively well studied. Only in the second half of the fourth century did Christianity develop a message attuned to the ideology and value-system of the social elite that would attract many of them.

Christian diversity is partly due to major political transformations within the later Roman empire. Perhaps the most important of them was the growing split between the Western and the Eastern parts of the empire. After the death of Constantine, the political division of the empire responded to administrative expediency and the military exigencies of almost uninterrupted warfare on the Rhenish, Danubian and Persian borders. The political centre shifted to the East, to Constantinople. During the fifth century, various German nations filled the power vacuum in the West. Their Homoian churches punctuated the map of Roman Christianities in Italy, Gaul, Spain and North Africa.