When around 180 ce Bishop Irenaeus of Lyons wrote his Detection and refutation of gnōsis falsely so-called, known simply by the Latin title Adversus haereses (‘Against heresies’), he hoped to bring order to a confused situation. A bewildering number of ‘Christian’ groups and teachers offered interested persons salvation, often in the form of gnōsis (‘knowledge’ or ‘acquaintance’) with God. Yet the teachings and practices of these ‘Christians’ displayed an astonishing diversity on such issues as the nature(s) of God and the creator of this world and the content and interpretation of scripture. Irenaeus presented his readers with a powerfully simple way to make sense of these competing claims. There was, he argued, a single consistent Christian truth, deposited in a single church spread throughout the world in communities that could trace their heritage back to Christ and his original apostles. All other groups that claimed to be Christian, despite their seemingly infinite variety, in fact were manifestations of a single error, false gnōsis, which originated in a single teacher, Simon Magus (Acts 8:9–24). The clarity of Irenaeus’ vision is so compelling that even today, after more than a century of scholarship undermining it, we moderns must exert great pains to see the Christianities of the second century in any other way.