The Old Testament (OT), according to the Church, reveals the one true God, and God so loved the world, we learn in the New Testament (NT), that He became man (John 3:16). A dangerous heresy called Gnosticism, among other things, separated the god of the OT from the god of the NT; indeed, Gnostics saw the material world that God created as evil. Irenaeus recognized that this gnostic heresy, which denied the humanity of Christ, was leading souls astray.
Irenaeus’ work, Against Heresies, meticulously and effectively dismantles the gnostic heresy. Irenaeus accomplishes this by, among other things, showing that Jesus Christ can actually be found in the OT, and, as the disciple John points out, “[i]n the beginning was the Word…[and] all things came to be through him…” In other words, God and Jesus Christ are one and the same.
It is understandable that some Christians would try to devise a logical explanation for the apparent change in the God of the OT and the God of the NT. “An eye for an eye,” became “turn the other cheek,” for example. Irenaeus’ explanation is not that God changed, for He (the Truth) is “impassible.” Irenaeus might refute the gnostics by pointing out that yes, we change, and thus in God’s mercy and love, the method of His incarnational revelation also changes. In His patience and mercy, He chose the time, the people, and the place for the fulfillment of the OT.
Belief in Christ saves souls, but only a belief in the true Christ—thus Irenaeus’ work, which helped to stop a serious heresy, is a great contribution by a great saint who devoted his life to the salvation of souls. His work is a tribute to the true faith for which so many have been martyred—and continue to be martyred.