To engage the question of Christian self-definition is to become keenly aware that it is a process of differentiations and negotiations that is never final, and that the categories of description – ‘Christian’, ‘Jewish’, ‘Greek’, ‘Roman’ – are not to be taken for granted. The communities these categories are said to designate are neither stable nor essentially known entities, but social formations continuously engaged in self-recreation. With this in mind, I have endeavored to analyse a crucial moment in the second century of the formation of a ‘Christian’ discourse and, indeed, of the construction of ‘Christianity’ itself. Justin Martyr was not the first to take up the question of self-definition vis-à-vis the Graeco-Roman world, and he would certainly not be the last, but he was surely one of the most influential to do so. It was Justin more than anyone else who would set the terms in which Christianity would be represented to the wider world of antiquity, and a whole host of Christian writers would follow in his path, elaborating and expanding upon his project of self-definition. What is more, a least one ‘Graeco-Roman’ author of the second century appears to have taken notice of Justin’s works and felt compelled to issue a response: the Alēthēs logos of the otherwise unknown Platonist, Celsus, represents the first systematic attack on Christianity. Taken together, Justin and Celsus signal a turning-point in the construction and contestation of Christian discourse in the second century.