In August 1535, Alexander Alesius (1500–65) left Wittenberg, where he had gone into exile a few years earlier, to return to Great Britain, albeit to England rather than his native Scotland. Formerly an Austin canon of St Andrew’s cathedral priory, Alesius had first made a name for himself by using John Fisher’s Assertionis Lutheranae confutatio to argue persuasively against Luther’s teachings. However, his assignment to persuade Patrick Hamilton to recant his Lutheran beliefs in 1528 had quite the opposite effect. Alesius was the one who privately changed his commitments. Soon after, the archbishop appointed him to address a provincial synod of bishops and clergy. Avoiding controversies in doctrine, he instead criticised the lax moral state of the clergy and, by implication, the well-known improprieties of his own prior, a man who eventually had ten illegitimate children and whose hand had joined those signing Hamilton’s death sentence.

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’.