Irenaeus of Lyons, a significant theologian of the second century AD, played an influential role in shaping the Christian understanding of the gospel, Scriptures, and church structure. While not a founder, his articulation provided an unparalleled scope that laid the groundwork for future church leaders. This volume, part of the Patristic Essentials series, situates Irenaeus in his historical context and emphasizes his contributions to second-century Christian doctrine. By including well-known passages and preserving their internal logic, it offers readers a clear understanding of his theology. Each chapter begins with an overview, supplemented by strategic headings, to guide readers through Irenaeus's arguments. Updated English translations are provided to aid contemporary readers. Through his careful curation and clear editorial guidance, Spellman introduces Irenaeus's vision of God's redemptive work to a new generation, reflecting the ancient theologian's comprehensive approach to Christian understanding.