Let the inspired Scriptures then be our umpire, and the vote of truth will be given to those whose dogmas are found to agree with the Divine words (On the Holy Trinity)But this quotation is rather badly torn from its context. Immediately preceding the given quotation, St. Gregory writes:
Now they charge us with innovation, and frame their complaint against us in this way:—They allege that while we confess three Persons we say that there is one goodness, and one power, and one Godhead. And in this assertion they do not go beyond the truth; for we do say so. But the ground of their complaint is that their custom does not admit this, and Scripture does not support it. What then is our reply? We do not think that it is right to make their prevailing custom the law and rule of sound doctrine. For if custom is to avail for proof of soundness, we too, surely, may advance our prevailing custom; and if they reject this, we are surely not bound to follow theirs.So we see that in the conflict of which he writes (a conflict concerning the doctrine of the Holy Trinity), he found himself at loggerheads with his adversaries: he would not accept their "custom", nor they his. As a consequence, if there was to be any means of resolving the conflict, it could not be done by means of a resort to their "custom" or his own. Well then, St. Gregory writes, since a resort to custom will not be possible in the present case, we should resort to Scripture as the "umpire". Far from having appealed first of all to Scripture (although no Catholic would object to this), it appears that St. Gregory actually would have appealed to "custom" first, if possible.
What is "custom" in the present context? It seems indistinguishable from tradition. That which is customary is that which is traditional. So what he appears to be saying is that he would certainly not accept the traditions of his opponents, and they reject his Tradition. In any event, it seems quite clear that it is absurd to use St. Gregory's appeal to Scripture here as supposed evidence that he believed in the Protestant doctrine of sola Scriptura.
And there is more:
it is enough for proof of our statement, that the tradition has come down to us from our fathers, handed on, like some inheritance, by succession from the apostles and the saints who came after them. They, on the other hand, who change their doctrines to this novelty, would need the support of arguments in abundance, if they were about to bring over to their views, not men light as dust, and unstable, but men of weight and steadiness: but so long as their statement is advanced without being established, and without being proved, who is so foolish and so brutish as to account the teaching of the evangelists and apostles, and of those who have successively shone like lights in the churches, of less force than this undemonstrated nonsense? (Against Eunomius, IV, 6; emphasis added)And:
The question is, as I said, very difficult to deal with: yet, if we should be able to find anything that may give support to the uncertainty of our mind, so that it may no longer totter and waver in this monstrous dilemma, it would be well: on the other hand, even if our reasoning be found unequal to the problem, we must keep for ever, firm and unmoved, the tradition which we received by succession from the fathers, and seek from the Lord the reason which is the advocate of our faith: and if this be found by any of those endowed with grace, we must give thanks to Him who bestowed the grace; but if not, we shall none the less, on those points which have been determined, hold our faith unchangeably (On "Not Three Gods"; emphasis added).So we see that St. Gregory of Nyssa also affirmed Sacred Tradition's authority, and that he was by no means an adherent of sola Scriptura.