Against Heresies (Adversus Haereses) is a five-volume work written by Irenaeus, the Bishop of Lyon in the late 2nd century, to combat the teachings of Gnosticism, a significant heretical movement at the time. Gnostic groups promoted complex doctrines that viewed the material world as inherently evil, created by a lesser divine being, and saw salvation as attainable through secret knowledge (gnosis) rather than faith in Christ. In his work, Irenaeus systematically refutes Gnostic beliefs by affirming the goodness of creation and the oneness of God as revealed in Scripture. He provides a defense of orthodox Christian theology, focusing on the unity of the Old and New Testaments and emphasizing the incarnation, where Christ is seen as central to God's plan for human redemption. Against Heresies not only offers insight into early Christian theological disputes but also remains one of the most influential works for understanding early Christian doctrine, the role of apostolic tradition, and the development of Christian orthodoxy.