While Athanasius' works are quote mined by some Protestants to show he believed in the formal sufficiency of the Bible, Athanasius held a view of the authority of councils that are opposed to various Protestant formulations thereof. In his To the Bishops of Africa (Ad Afros), he clearly believed that ecumenical councils had an intrinsic, binding authority upon Christians; they were not binding simply by their fidelity to the Bible. Note the following quotes and the original language text thereof:
The confession arrived at Nicea was, we say more, sufficient and enough by itself for the subversion of all irreligious heresy and for the security and furtherance of the doctrine of the Church (To the Bishops of Africa, 1)
PG 26:1029:
Notice that Athanasius uses αυταρκη to describe the 'sufficiency' of Nicea; this is the same term used of Scripture by Athanasius in his On the Incarnation of the Word, 56. For Athanasius, Nicea was just as "self-sufficient" as the Bible!
But the word of the Lord which came through the ecumenical Synod at Nicea, abides forever. (To the Bishops of Africa, 2)
PG 26:1032:
Here, Athanasius says that the word (ρημα) of the Lord came through Nicea, and it "abides for ever" (με νει εις τον αιωνα). Athanasius is borrowing from the language of Isa 40:8 LXX:
τὸ δὲ ῥῆμα τοῦ θεοῦ ἡμῶν μένει εἰς τὸν αἰῶνα ("the word of our God remains forever" [NETS])
No Protestant would ever say such things about any source of truth/authority other than inscripturated revelation, but Athanasius explicitly imputes the authority of inscripturated revelation to the council of Nicea. Athanasius was clearly not a Proto-Protestant.
Further Reading
Not By Scripture Alone: A Latter-day Saint Refutation of Sola Scriptura