To Alcuin, as to his predecessors in the seventh and eighth centuries, this 'truth of the church' was that which had been defined by the fathers of the church. What has been called 'the self-effacement of Alcuin in relation to the fathers' was the dominant theme and method of his theology. In all questions of doctrine, he said, 'I wish to follow the footsteps of the holy fathers, neither adding to nor subtracting from their most sacred writings.' ... Therefore one would not err if one resolved to abide 'in the company of such men within the camp of the catholic faith,' for outside that camp were the enemies of the faith. [Pelikan, The Growth of Medieval Theology, 15]This conservative and reverent attitude toward the Faith they had received is perhaps most obvious if we take note of the single most important text of the Middle Ages (excepting the Bible): Peter Lombard's Book of Sentences. This work consists of quotations (in large part glosses on the Bible) compiled by Lombard from the Church Fathers; as the Catholic Encyclopedia article says, "scarcely more than ten lines have been found to be original" to Lombard himself. Lombard's work was the standard theology text for four centuries.
Now whatever else one might say about it, an age in which an appeal to authority was fundamental to the pursuit of the truth can hardly be said to be innovative. So it seems, even if we were inclined to ignore the weight of Mt. 16:18 ("the gates of hell shall not prevail against" the Church), that the likelihood of the Church going astray and losing or abandoning or rejecting the truth of the Faith ought to be viewed as vanishingly small. But when we take into account God's promise in Mt. 16:18 that the Church will never be overcome by Hell, we see that the Medieval attitude towards the Fathers really seems to be a providential outworking of that promise.
But there is more to what the Medievals did than simply appealing to the Fathers. If they did only this, they would really be doing little more than observing the later method of the Protestants' sola scriptura. But they did not do this. Instead, they anticipated the rule set down in CCC §113 (and repeated here frequently): "Read the Scripture within the living tradition of the whole Church." We must read the Church Fathers the same way, and this is what theologians of the Middle Ages did.
Ambrose Autpert said, 'one should not be prejudiced by the period in which he wrote, but should seek to discern only whether what he wrote is true and catholic or false and heretical.' ... Bede's spiritual descendant, Alcuin of York, proudly identified himself as 'a catholic, born and reared in the catholic church,' and he was convinced that apart from the 'truth of the church' there could be no salvation. [Pelikan, op. cit., 14-15]It wasn't (and isn't) enough merely to say that such-and-such Father taught this; on the contrary, what the Fathers say must be measured by the standards of the Tradition of the Faith as taught by the Church. Consequently they complain baselessly who grumble that Catholics do not identify Sacred Tradition with the writings of any Father. They either misunderstand what we believe, or they are arguing against a straw man.
Why were the schoolmen so violently opposed to theological novelty?
The apostolic anathema pronounced against anyone, even 'an angel from heaven,' who preached 'a gospel contrary to that which you have received' by tradition was, as in the East so also in the West, a prohibition of any kind of theological novelty. ... One who denied the consensus of such catholic doctors on [for example] the doctrine of Mary was dismissed as 'the fabricator of a new error.' [Pelikan, op. cit., p. 17]Questions come to mind at this point. If (as some critics allege) the Catholic Church has "lost" certain critical parts of the Gospel, and has accumulated a raft of theological novelties for itself (as they claim) - if these things are so, in spite of God's promise in Mt. 16:18, and in spite of the demonstrated zeal of the theologians of the Middle Ages for preserving and adhering to the truth of the Gospel - then why should we believe that others (usually the critics themselves) have discovered these missing truths and have faithfully preserved them? What zeal for the truth have they got that was lacking among Catholics? Or how is it that the Holy Spirit abandoned the Catholic Church but (as is usually alleged) has blessed the rediscoverers of Truth? Their claims simply don't work, it seems to me.