How the Church has historically determined doctrine is a poignant question, especially as it relates to such issues as Mary’s role in salvation, Papal authority, apostolic succession, and the nature of the Eucharist. This would equally apply to soteriological matters. When we ask the question, however, we soon discover that it is not enough to make singular and direct appeal to any one source, be it Scripture alone, the pious actions of the Church at a given point in history, or the witness of a solitary Father. Rather, as Jaroslav Pelikan makes clear, there must be a consensus among the Fathers of the Church, in concert with Sacred Scripture, by which we can be sure that doctrine is true.

The true and authentic consensus was that which reflected the mind of the Catholic and Universal Church […]. Patriarch Sophronius of Jerusalem, a contemporary of Maximus, summarized this idea of patristic consensus in a similar way: “An apostolic and ancient tradition has prevailed in the holy churches throughout the world, so that those who are inducted into the hierarchy sincerely refer everything they think and believe to those who have held the hierarchy before them. For…all their running would be in vain if an injustice were to be done to the faith in any respect” (Soph. Ep.syn. (PG 87:3149-52). Sophronius’s formula, “an apostolic and ancient tradition,” did not mean that everything “ancient” was therefore automatically “apostolic.” All the orthodox theologians knew that in some instances “antiquity means foolishness.” Even Irenaeus had erred in teaching the idea of the millenium. But while all that was ancient was not apostolic or orthodox, all that was orthodox had to have been apostolic and was therefore ancient. True doctrine, as Theodore of Studios was to assert, was “the excellence of the apostles, the foundation of the fathers, the keys of the dogmas, the standard of orthodoxy,” and anyone who contradicted it, even if he were an angel, was to be excommunicated and anathematized.

from J. Pelikan, The Spirit of Eastern Christendom, 600-1700 (The Christian Tradition series, Vol. 2), University of Chicago Press, Chicago, 1974, p. 22.​