St Irenaeus pretty much saw evil as the Fall from Grace by Adam and Eve, which God responds to by sending Jesus. This is called the doctrine of Atonement in the Christian Churches.
Irenaeus sees salvation as essentially coming about through the incarnation of God as a man. He characterises the penalty for sin as death and corruption. God, however, is immortal and incorruptible, and simply by becoming united to human nature in Christ he conveys those qualities to us: they spread, as it were, like a benign infection. Irenaeus therefore understands the atonement of Christ as happening through his incarnation rather than his crucifixion, although the latter event is an integral part of the former.
Irenaeus wrote Against the Heresies, one of the earliest theological texts in the history of Christendom; it was written against the Gnostic Heresy.