See also the article in the Catholic Encyclopaedia.

  Whenever we take up a Bible we touch Irenaeus’s work, for he played a decisive role in fixing the canon of the New Testament. It is easy for people nowadays to think of Scripture – and the New Testament in particular – as the basis of the Church, but harder to remember that it was the Church itself that had to agree, early on, about what was scriptural and what was not. Before Irenaeus, there was vague general agreement on what scripture was, but a system based on this kind of common consent was too weak. As dissensions and heresies arose, reference to scripture was the obvious way of trying to settle what the truth really was, but in the absence of an agreed canon of scripture it was all too easy to attack one’s opponent’s arguments by saying that his texts were corrupt or unscriptural; and easy, too, to do a little fine-tuning of texts on one’s own behalf. Irenaeus not only established a canon which is almost identical to our present one, but also gave reasoned arguments for each inclusion and exclusion.

  Irenaeus also wrote a major work, Against the Heresies, which in the course of denying what the Christian faith is not, effectively asserts what it is. The majority of this work was lost for many centuries and only rediscovered in a monastery on Mount Athos in 1842. Many passages from it are used in the Office of Readings.