The influence of an arch-heretic In a period when Christianity was only beginning to form a definitive identity, Marcion played a remarkable and generative role. Andrew Hayes takes the measure of Marcion’s impact on second-century Christianity through a close examination of the topics and structure of Justin Martyr’s writings, especially Dialogue with Trypho, demonstrating that Justin repeatedly described Christianity in a contra-Marcionite fashion. Arguing that the early part of Dialogue is, in fact, a contra-Marcionite prelude to all the major themes in the rest of the piece, Hayes claims that the chief task Justin took for himself was to seize back from Marcion the terms of Christian self-definition. Marcion is thus far more important for Justin’s work than the sparse explicit mentions might suggest, and Hayes shows that these texts are far from anomalous: they reveal Justin’s deeper agenda of presenting Marcion as a demonic instrument. Students of the second century, of Marcion, and of Justin alike will find much to reevaluate in these pages.