Theological Contribution
In his five-volume work against the heresies, Irenaeus answered the Gnostic teachers of his age, above all Valentinus and Marcion, who divided the God of creation from the God of redemption and disparaged the material world. Against them he insisted on the unity of God, the goodness of the creation, and the reality of the Incarnation, by which the Son of God took on human flesh born of a Virgin.
He is remembered as an early and influential witness to apostolic succession, holding that the unbroken line of bishops from the Apostles safeguarded the true teaching of the Church. He was also among the earliest writers to regard the four gospels together as the foundational Scripture, and he developed the theme of recapitulation, in which Christ as the new Adam reverses the disobedience of the first Adam through his own obedience.