If we wish to have an eye out for reasoning from conclusions, RC apologists do that all the day long, filtering out ALL which does not support their own conclusions.
There is much focus upon the early heresies of such as Marcion, but there and against other heresies we find the church reliant upon what? Not their own authority, but the fullness of the Gospels (all four books, not just Luke), the complete collection of Pauline Epistles, and Acts.
None of the churchmen who argued Chritian theology from those collected works, putting down those whom would accept only lesser NT canon, or add to it gnosticisms, themselves wrote that which they relied upon. Instead, they held up a standard for what by them had been recieved.
If Dunbar confirms the authority of the capital "C" Church (which can ONLY be the Catholic Church in your own eyes, correct?) then upon what does that authority rely, but upon those written works they themselves recieved, handed down over several centuries before the process of canonization was forced by the existence of heresies? What did the church combat those heresies with, but with those works which can indeed be shown to have come from the Apostles? That is what Dunbar was referencing in the below;
From this perspective the sharp reaction of the Fathers to Marcion and the Gnostics is to be seen, not as a de novo selection of an alternative canon, but rather as a making explicit of what had always been implicit in the life of the Church.
If there is confirmation of authority, it was reliant upon (what eventually came to be known as) the NT writings, and how such was understood then. Departures from those texts --- not allowed. Interpretations of those texts --- forever open to inspection themselves, or else there arising Sola Ecclesia, a "whatever we tell you" approach. We can see that in Luther's day, that sadly limited "sola" could itself produce heresies within the Church. Looking around and finding a few grotesque errors, Luther kept looking and found more fault with sola ecclesia, he himself being an ecclesiastic, showing himself in process to be more concerned with recovering what was originally recieved, but less interested in the ecclesiastical organization which was clearly enough shown at that time to have departed from the very basics of the Gospel message found within the texts.
There is no disputing by me that the Fathers (you call them “churchmen, right?) relied on Scripture. I emphasize that they did not speak in isolation or merely claim their thoughts were obvious from a plain reading of Scripture vouchsafed by the Holy Spirit - period, end of story.
Rather, they asserted that their beliefs revealed and defined with ever greater precision, what had been delivered to the universal church beginning with the Apostles and handed down by faithful men.
(I am talking first millennium. See my post 275 regarding what I will not respond to. Catholic is not capitalized in that world.)