Conversion, as a recognised phenomenon in the history of religion, is generally considered to manifest itself in two fairly distinct, albeit interrelated, ways. One is personal, and the outcome of individual agency (or, arguably, divine grace); the other social and political, and produced or facilitated by the (often coercive) exercise of earthly power. Thus, we speak about the conversion of St Augustine or Luther's conversion, and also about the conversion to Christianity of Anglo-Saxon England or Scandinavia's conversion to Protestantism in the course of the sixteenth century. The former mode of conversion is an intense, usually private, spiritual experience of transformation and renewal – of turning or reorientation. Historically, it is one which we can recognise as a conversion only when it is performed, or narrativised in some way. The latter form of conversion might well involve such dramatic affirmations in particular cases, but it need not do so, and it should certainly not be considered as the sum or aggregate of individual conversions in a Pauline or Augustinian sense. The processes involved here have often been referred to as the ‘Constantinian’ model of conversion, rather than the Augustinian. There is in fact debate among scholars as to whether ‘conversion’ is at all the appropriate designator when societies as a whole transfer allegiance from one belief system to another, nearly always as a result of an exertion of political might. Some historians, whether of late antique Europe, or of the early modern Americas, choose to speak of Christianisation rather than conversion. Others prefer the apparently oxymoronic term ‘forced conversion’, though the degree of force required varied considerably across different historical settings. Many historians studying the effects of these societal, political conversions now emphasise patterns of social and cultural negotiation, of gradual adaptation of new norms, which are changed in the process of being received. In a recent overview, commenting on work about the first centuries of Islamisation in Iran, Felipe Fernandez-Armesto states that ‘adaptation occurs independently of conversion, and … over time the religious profile of a society can change without much impact on the individuals who compose it’.

The era of the Reformation witnessed an intensification of both forms of conversion, within Europe and also beyond it.