St. Athanasius (c. 297-373) was a great Church Father and heroic opposer of the heresy of Arianism. He is probably the most-cited Church Father after St. Augustine in the writings of Protestant apologists who wish to show that the fathers were closer in substance to various teachings to Protestantism than to Catholicism.

He is cited as a supposed proponent of Sola Scriptura — the Protestant rule of faith and notion that the Bible is the only infallible source and standard for theology. It follows logically from this definition that the Church (including ecumenical councils) or sacred tradition (including apostolic succession) cannot be infallible sources or standards for theology.

Therefore, if someone asserts that one or both are infallible sources, then by definition and logic that person cannot possibly adhere to Sola Scriptura. It's rather easy to demonstrate that St. Athanasius did indeed believe in infallible sources of authority alongside, and in harmony with Sacred Scripture.

I cite his words from the 38-volume edition of the Church fathers edited by Philip Schaff (available online in its entirety at the New Advent website):

• “But the word of the Lord which came through the ecumenical Synod at Nicaea, abides forever.” (Ad Afros Epistola Synodica 2)
• “But let the Faith confessed by the Fathers at Nicæa alone hold good among you, at which all the fathers, including those of the men who now are fighting against it, were present, as we said above, and signed: in order that of us too the Apostle may say, ‘Now I praise you that you remember me in all things, and as I handed the traditions to you, so hold them fast (1 Corinthians 11:2).’” (Ad Afros Epistola Synodica 10)
• “For had they believed aright, they would have been satisfied with the confession put forth at Nicæa by the whole Ecumenical Council; ... Observe how entirely they disregard the truth, and how everything they say and do is for the sake of the Arian heresy. For in that they dare to question those sound definitions of the faith, and take upon themselves to produce others contrary to them, what else do they but accuse the Fathers, and stand up in defense of that heresy which they opposed and protested against?” (Ad Episcopus Aegypti et Libyae, 5)
• “Who, then, that has any real regard for truth, will be willing to suffer these men any longer? Who will not justly reject their writing? Who will not denounce their audacity, that being but few in number, they would have their decisions to prevail over everything, and as desiring the supremacy of their own meetings, held in corners and suspicious in their circumstances, would forcibly cancel the decrees of an uncorrupt, pure and Ecumenical Council?” (Ad Episcopus Aegypti et Libyae, 7)
• “It is enough merely to answer such things as follows: we are content with the fact that this is not the teaching of the Catholic Church, nor did the fathers hold this.” (Letter No. 59 to Epictetus, 3)
• “What defect of teaching was there for religious truth in the Catholic Church …?” (De Synodis, I, 3)
• “… the sectaries, who have fallen away from the teaching of the Church, and made shipwreck concerning the Faith.” (Against the Heathen 1, 6, 3)
• “... the soul is made immortal is a further point in the Church’s teaching which you must know ...” (Against the Heathen 2, 33, 1)

In seeking to establish that a particular Church Father believed in Sola Scriptura, Protestant apologists will often cite passages in their writings having to do with the material sufficiency of Scripture: which means that Scripture contains everything sufficient for one to be saved. But Catholics and Protestants agree on that doctrine, so it’s irrelevant to the debate about Sola Scriptura. The Church Fathers — almost to a person, as I have discovered in my own research on the topic – reject Sola Scriptura, which is the same as the formal sufficiency of Scripture. Invariably they do so in the sense that they believe there are infallible sources of authority and theology alongside the Bible.

Another error and fallacy that is very common in these sorts of Protestant treatments of Church Fathers is the belief that if a Father cites a lot of Scripture in his argumentation (and not infrequently only Scripture), that he must, therefore, believe in Sola Scriptura.

This doesn’t follow at all. It’s two different things; apples and oranges. It’s undeniably the case that one doesn’t have to believe that only Scripture is infallible in order to use Scripture in theological argument. I massively cite Scripture, myself, in my many apologetics articles and books. One of my specialties, and what I am most known for, is “biblical evidence for Catholicism.” Yet I vehemently deny Sola Scriptura, and have written three books against it.

St. Athanasius could (and did) make many arguments from Scripture alone. But he also made arguments of the authority of Tradition or councils alone, or from an appeal to apostolic succession alone. In his statements about the Council of Nicaea (above), he clearly didn’t think that it erred at all in its pronouncements, or (so it seems to me) even that it could possibly assert error. Many times, Athanasius mentioned the infallible authority of both Scripture and non-Scripture in the same context. For example, he wrote about Scripture:

But ... if you light upon the text of the Scriptures, by genuinely applying your mind to them, will learn from them more completely and clearly the exact detail of what we have said. For they were spoken and written by God, through men who spoke of God. (On the Incarnation of the Word, 56, 1-2)

But this is not Sola Scriptura, because the very next thing he wrote contradicts it: “But we impart of what we have learned from inspired teachers who have been conversant with them ...” (Tradition and apostolic succession).