A quick justification for the title would simply be a thousand years is as a day to the Lord, etc. Thus, for masses of the church to have erred for thousands of years is but a day’s folly in the eyes of God.
However, a more complete answer is that theology is not sociology nor a matter of opinion polling. The determination of theological truth ultimately has to be logically independent sociological factors like how long a belief was held or how widespread was a belief. Cicero’s criteria for evaluating the trustworthiness of testimonial evidence needs care, and we need to be especially critical of the criteria of how widespread or ancient a belief is as evidence of truth. As Cyprian of Carthage would put it: “Custom without truth is the antiquity of error”.
Furthermore, the criteria for “how long is too long” is completely arbitrary, there is no magical number of years whereby we can determine when a theological matter was settled or unsettled. When the Roman Empire officially adopted a Semi-Arian Creed in 360 until 381, was 21 years a long period of time? When the Church did not codify the express formulates of one ousias in three hypostasis for three centuries, is 300 years a long time? What constitutes a “long time” to be without an explicit codification or expression of the truth? In 100,000 years Christians looking back at our mere 2,000 years of history would judge us to be merely a blip in the totality of 100,000 years of church history. There’s nothing magical about some number of years which a belief has been held.
What if we were to discover that for a majority of the Church’s time were mistaken about the beginning of life and the status of abortion? What if we were to dig up St Paul’s letter to the Laodiceans? Should we reject all that simply because a vast majority of the Church were without it for 2000 years? Or more interestingly, should we in principle reject the Ethiopian Orthodox expanded New Testament canon simply because the rest of us do not have it for so long?
Here’s another hypothetical scenario. Suppose there was a tribe off the coast of India who had once been visited by a Christian missionary over two thousand years ago. However that missionary only left behind several books of the Bible, but not the fuller set. Say, maybe it’s missing a lot of the minor prophets, maybe it only has 2 gospels and some letters of John were missing. Suppose for two thousand years this island was undisturbed, but the few who converted spread the Word and converted the whole island, and they were undisturbed for two thousand years until today some Christian explorer went to the island. Suppose we looked at their “bible” and note that some books were missing and we said, we have the full canon here. Should these hypothetical islanders reject our larger canon simply because they have been using theirs for two thousand years? Or should they not instead receive with joy the fuller divine word and revelation which God has now condescended to give them? Should we not then have the same attitude towards whatever new theological truths or insights we may have today over our forefathers in faith?
In these disorienting times the temptation is just “to go back” and ape what the past did. But there is a reason why the past lead to the present, and ultimately an appeal to the past just because the past is the Burkean error of sanctifying presentism I’ve criticised constantly. It is more important to seek out the ratio for the past rather than just adhere to the past, and accept it only if the ratio is acceptable, otherwise not. Christianity was once new, fresh, and unprecedented. It was accepted not because it was old but because it was the Word of God. The Gospel truth must be renewed in every age and ever time by an appeal directly to the will and Word of the ever-present God, and it is ever fresh, ever new, and ever contemporary in every age.
In this we must avoid what Donald Mackinnon once said about ecclesiological fundamentalism:
Theological progress may be dependent upon the criticism of the Church’s institutional experience, even the rejection of long tracts of that experience as fundamentally invalid. In such criticism may well lie the necessary condition of really fundamental theological progress…
And we should also take a leaf from the common sense Scottish philosopher James Beattie concerning how to evaluate the times, both past and present:
They who form opinions concerning the manners and principles of the times, may be divided into three classes. Some will tell us, that the present age transcends all that have gone before it, in politeness, learning, and good sense ; will thank Providence (or their stars) that their lot of life has been cast in so glorious a period; and wonder how men could support existence amidst the ignorance and barbarism of former days. By others we are accounted a generation of triflers and profligates; sciolists in learning, hypocrites in virtue, and formalists in good-breeding; wise only when we follow the ancients, and foolish whenever we deviate from them. Sentiments so violent are generally wrong; and therefore I am disposed to adopt the notions of those who may be considered as forming an intermediate class; who, though not blind to the follies, are yet willing to acknowledge the virtues, both of past ages, and of the present. And surely, in every age, and in every man, there is something to praise, as well as something to blame.