He quotes the exact words of Marcion and Valentinus and other heretics as proof of what he says that they believe. He doesn't ask the reader to take his word for it. He cites their exact words from their "written documents".
Yet when it comes to telling us that Peter and Paul ordained the Church of Rome with superiority, he cites neither Peter nor Paul nor any other apostle. He cites no "written document" as evidence. The reader is supposed to take his word for this outlandish claim.
On the one hand he accuses the heretics of resorting to "tradition" or "viva voce" in lieu of the truth of the "written documents", and then resorts to the same thing himself.
Is it possible that the reason that he cites no source for his claim of superiority of the Church of Rome is because there was none and he knew it? The absence of any "written documentation" for his claim in light of what he had said in that regard of the heretics, goes to the heart of the lack of any substantive credibility for that claim.
We are not saying he is *the* authority, but that he is *an* authority. If we saw, for instance, that half the Fathers said this about Roman primacy, and the other half said the opposite, well then yes, you'd have a point.
But we don't see that at all. Look through everything the Fathers said on this score. You find that some Fathers assert the primacy of Rome--some more clearly, some less clearly. Some are silent on the matter. But NO Father denies it. No one rose up in the 300s and said Irenaeus was dead wrong. Or the 400s. Or the 500s. Or the 600s. Or the 700s.
So no, I do *not* believe that everything Irenaeus said has to be 100% accurate. But you are asking me to believe that what he said was not accurate on the strength of your opinion alone. If you can find me a Father that refutes him, directly or indirectly, you will have a much better leg to stand on. Until then, I submit that he had a better view of what was orthodox doctrine in the mid-100s than you or I do.
Kinda causes problems when someone who knew someone who knew an apostle tells you that the See of Rome is the touchstone of orthodoxy, huh?
Maybe most people within the church at the time didn't consider his claim to be particularly "outlandish". Maybe they considered it to be something closer to common knowledge. We certainly have no record of anyone at the time, especially anyone with a more impressive pedigree of Christian discipleship, saying, "Irenaeus, you ignorant fool! Whatever caused you to come up with such an outlandish idea?!?"