The Introduction presents the main topic of the book – the role of conversion and competition between Nicene and Homoian churches in the post-Roman West – and the methods applied. It explains terminology (Nicene, Homoian), theorises the concept of conversion as a tool of historical analysis, and presents the purpose of the cross-regional comparison that follows.

Locke omitted the Trinitarian dogma from his elucidation of the Christian religion in "The Reasonableness of Christianity." This omission implicitly made belief in the Trinity unnecessary to salvation and attracted much criticism, leading John Edwards and others to accuse Locke of Socinianism. Locke refused to clarify his position on the Trinity even when Edwards and Stillingfleet pressured him to do so. His public silence on the Trinity was surprising to many, because the "Reasonableness" appeared in the middle of the heated Trinitarian controversy of the late seventeenth century. Locke actually expressed, unsystematically and at times ambiguously, his views on Christ’s nature and mission in his public writings on religion and in various manuscript notes, and he focused on Trinitarian issues in "Adversaria Theologica" and other manuscripts. His Christological reflections and his consideration of Trinitarian issues denote a heterodox, non-Trinitarian conception of the Godhead, which presents both Socinian and Arian elements, although he never expressly denied the Trinity. Irenic and prudential reasons contributed to his choice to avoid public discussion of the Trinitarian dogma.