This paper seeks to evaluate the ecclesiology of St. Irenaeus, bishop of Lyons, particularly as it relates to his defense of orthodoxy against false teaching in his five-volume work, Against Heresies. Irenaeus employs sound reason and biblical theology in his battle against Valentinus, Marcion, and other heretics. But Irenaeus also shows himself to be not only a theologian but also a devoted churchman. In all five books, his teaching on the church plays a significant role in exposing and refuting the false Gnostic teaching of his day. This paper will argue that Irenaeus employed his ecclesiology to distinguish between Gnosticism and the apostolic teaching in Against Heresies. He did this in two ways. First, he argued for the doctrinal identity of the church. The true church is built on apostolic teaching, and any deviation from that teaching results in a false church. Second, he argued that the church’s “ecclesiastical constitution,” both in her presbyters and her members, embodied the apostles’ teaching. In all this, the church preserved the truth entrusted to her by Christ and his apostles.
