Christianity is, as Diarmaid MacCulloch has observed, not so much a ‘religion’ in the conventional sense as a personality cult: an extraordinarily diverse and quarrelsome family of faiths which agree on almost nothing except putting the figure of Jesus of Nazareth at their centre. But there are a handful of other features very widely shared across that family, one of which, ironically enough, is the concept of orthodoxy: literally, ‘right opinion’. As historians, it is not our role to assess which doctrinal truth-claims are right; nor can we responsibly assume that they are all wrong. We can, however, observe that Christians see the matter as profoundly important. They may never have been able to agree on the doctrinal expression of their beliefs, but they have generally agreed that they ought in principle to be able to do so. The notion that there is a ‘catholic’, or universal, Christian faith – that which, according to the famous fifth-century formula, has been believed everywhere, at all times, and by all people – is itself an act of faith: to reconcile it with the historical fact of persistent division and plurality requires a ruthless willingness to exclude some self-professed Christians, or an indulgent readiness to include others, or (more commonly) some combination of the two. And yet, if Christian orthodoxy and the one holy, catholic and apostolic church which is its custodian are more visible to the eye of faith than to the historian, historians cannot avoid the persistent power of the ideal of orthodoxy, which Christians have never (yet) attained but for which they have always strived. This volume is about the pursuit of that ideal and its consequences for the history of Christianity.

This gap between a universally recognised Christianity throughout all ages and places and the myriad competing conventions of its widely diffuse local embodiments is so persistent that it seems to be not an aberration but part of the very nature of Christian faith itself. The missiologist Andrew Walls has argued that the Incarnation was a ‘divine act of translation’. The Word becoming flesh accepted one local human culture as suitable for the particularisation of universal truth. Ever since, each Christian community has believed that in Scripture ‘God is speaking to its own situation’.