This chapter analyses 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. Taken together, Justin and Celsus signal a turning-point in the construction and contestation of Christian discourse. Justin's notion of an ancient Mosaic philosophy on which the Greek philosophers depended betrays the influence of contemporary ideas about the history of philosophy. The distortions and corruptions with which contemporary Platonism was riddled could be compensated for through a process of triangulation among the doctrines of Plato, the precepts of Pythagoras and the institutions of the most ancient 'barbarian' peoples. The feature of Greek philosophical theology was the attempt to reinterpret traditional religion in light of a form of monotheism.