While I have not seen any major other treatise explaining why Irenaeus was a major heretic (actually, outside of Church of God circles, I have little that specifically considers him to have been a heretic, though groups like the
Jehovah's Witnesses probably do), it appears to me that he may have been the most dangerous heretic.
Why?
Because, Irenaeus' heresies were not obvious to those outside the true Church of God. His heresies and false statements were less numerous and less obvious than Justin's, thus have been missed by nearly all scholars (though some have noticed one or two errors he made).
What were his most important heresies?
Irenaeus heard Polycarp, yet made deals with Rome that disagreed with Polycarp's teachings. Irenaeus knew that Justin did not agree with Polycarp, yet he approved of Justin--and sadly seemed to prefer to be influenced more by him that by Polycarp.
Irenaeus knew that Polycarp condemned heretics such as
Marcion and
Valentinus, but failed to mention that they were still allowed to be Roman Catholic until at two decades later. Irenaeus supported the Roman Church even though Irenaeus knew they tolerated heretics that had earlier been condemned by Polycarp (and eventually by Irenaeus himself).
Notice this from Tertullian:
Where was Marcion then, that shipmaster of Pontus, the zealous student of Stoicism? Where was Valentinus then, the disciple of Platonism? For it is evident that those men lived not so long ago,—in the reign of Antoninus for the most part,—and that they at first were believers in the doctrine of the Catholic Church, in the church of Rome under the episcopate of the blessed Eleutherus, until on account of their ever restless curiosity, with which they even infected the brethren, they were more than once expelled (Tertullian. The Prescription against Heretics, Chapter 30. Translated by Peter Holmes. Electronic Version Copyright © 2006 by Kevin Knight. All rights reserved).
Even though
Marcion and
Valentinus were condemned by
Polycarp as a heretic about two decades before
Eleutherius became bishop, apparently they were not put out of the Roman Catholic Church then. (Marcion gave a large financial contribution that kept him in good graces for a while--though the Roman Church allegedly returned that contribution after some time.)
And why is all of this about Polycarp and Irenaeus so important?
Because Irenaeus knew that Polycarp had the original faith that the apostles had, but did not stand up for it. Apparently, he did not consider that faith to be important enough to fully follow it personally. Notice what Irenaeus records this about Polycarp:
But Polycarp also was not only instructed by apostles, and conversed with many who had seen Christ, but was also, by apostles in Asia, appointed bishop of the Church in Smyrna…always taught the things which he had learned from the apostles, and which the Church has handed down, and which alone are true. To these things all the Asiatic Churches testify, as do also those men who have succeeded Polycarp down to the present time (Irenaeus. Adversus Haeres. Book III, Chapter 4, Verse 3 and Chapter 3, Verse 4).
So we have from this early Roman Catholic source that Polycarp and his successors in Asia Minor (at least until the time that Irenaeus wrote this, around 180 A.D.) practiced the true teachings that they learned from the apostles.
Irenaeus also wrote:
And Polycarp himself replied to Marcion, who met him on one occasion, and said, "Dost thou know me?" "I do know thee, the first-born of Satan."(Irenaeus. Adversus Haereses. Book III, Chapter 3, Verse 4. Excerpted from Ante-Nicene Fathers, Volume 1. Edited by Alexander Roberts & James Donaldson. American Edition, 1885. Online Edition Copyright © 2004 by K. Knight).
. . .
Irenaeus also had some teachings that Polycarp held. For example, like Polycarp, Irenaeus clearly did not teach the trinity--he held a
binitarian view, hence he did not follow that error of Montanus or Valentinus:
...there is none other called God by the Scriptures except the Father of all, and the Son, and those who possess the adoption (Irenaeus. Adversus haereses, Book IV, Preface, Verse 4. Excerpted from Ante-Nicene Fathers, Volume 1. Edited by Alexander Roberts & James Donaldson. American Edition, 1885. Online Edition Copyright © 2004 by K. Knight).
Notice that Irenaeus states that only the Father, the Son, and those who possess the adoption (Christians) are God. This is a binitarian, not a trinitarian view.
So why would binitarians consider Irenaeus a dangerous heretic?
Because he sided with Roman unity above the teachings that he must have known that Polycarp held. And he also introduced prophetic and other misunderstandings that many still accept today.
Now, notice one statement he wrote that Rome really likes (cf. #
834 Catechism of the Catholic Church, pp. 240-241):
...the very great, the very ancient, and universally known Church founded and organized at Rome by the two most glorious apostles, Peter and Paul; as also [by pointing out] the faith preached to men, which comes down to our time by means of the successions of the bishops. For it is a matter of necessity that every Church should agree with this Church, on account of its preeminent authority, that is, the faithful everywhere, inasmuch as the tradition has been preserved continuously by those who exist everywhere. (Irenaeus. Adversus haereses, Book III, Chapter 3, Verse 2)
But at this time in history, Irenaeus must have known the differences in Passover and the false prophet Montanus, etc. that the faithful in Asia Minor held from those of Rome. Furthermore, the Bible contradicts the account about Peter and Paul founded the Church of God in Rome. Additionally, even Roman Catholic scholars admit that Peter and Paul did NOT found the Church in Rome (for some details, please see
What Do Roman Catholic Scholars Actually Teach About Early Church History?). Yet, Irenaeus put out wrong information that is still be relied on today.
