This dissertation advances scholarship on textuality and knowledge in late antiquity through interdisciplinary analysis of the Eusebian apparatus. Devised by the fourth-century scholar Eusebius of Caesarea, the apparatus organizes an intricate web of textual relationships. The four Gospels of the New Testament (Matthew, Mark, Luke, John) share language, narratives, and ideas, yet they also differ in structure and detail. I argue that Eusebius employed emerging textual technologies to create new possibilities of reading, thereby rewriting the fourfold Gospel in a significant and durable way. The meaning of the Eusebian apparatus was, and continues to be, in its use—in the possibilities of textual encounter it affords and constrains. Eusebius' system has enjoyed an enormous reception, accompanying the Gospels in more than a dozen languages and in thousands of manuscripts from the fourth century CE to the present. By tracing how the Eusebian apparatus reconfigured Gospel literature, created new possibilities of reading, and developed in response to readers' changing priorities, I advance scholarship on textuality and knowledge in three major ways. First, I advance the historiography of the late ancient Mediterranean by demonstrating how material and textual technologies participated in a late ancient transformation in knowing. Second, I nuance conventional histories of Gospel literature by demonstrating the significance of fourth-century developments. Third, I demonstrate the paradoxical centrality of marginal phenomena in the history of Gospel reading and beyond.