The sixteenth-century Reformation wrought major changes, often underestimated, on the definition of key words and concepts in Christian theology. For centuries the Latin West had generally agreed on the idea of ‘orthodox’ belief and teaching – even if not on all aspects of its content. ‘Orthodoxy’ resided in the communities of faith belonging to the true and authentic Church. In the misleading but often-quoted phrase of St Vincent of Lérins (d. c. 450), the Catholic faith was ‘that which has been believed everywhere, always and by all’. Vincent devised this phrase precisely in order to define the Catholic faith against the abundance of past heresies, and to find a path through the exotic diversity of interpretations of canonical Scripture. Orthodoxy was social and political: it was what the authoritative voices believed and taught. Further, orthodoxy was for many centuries defined largely by its opposite, heresy. ‘Orthodoxy’ emerged from conflict: over many decades it eventually became accepted that (for instance) Arian views on the Trinity were ‘heretical’ and Athanasius’s views were ‘orthodox’. Nevertheless, by the late fourteenth century Jean Courtecuisse (d. 1423) could claim that every possible heresy had already been condemned either implicitly or explicitly.