Until the canon of the NT was collected together under one “cover”, the rule of faith, “the preaching”, “the tradition”, “the faith”, functioned as the standard for correct doctine.
Irenaeus, Tertullian, Origen, and Athanasius all had a “rule of faith” or “the tradition” and the content of all of it was completely Scriptural. Every one is full of scriptural truth, and they are all a kind of doctrinal statement that is similar to the Apostles creed and Nicene creed and other formal creeds, developed in the 300s and 400s. Not one of them in Irenaeus or Tertullian or Athanasius has anything in them in those contexts any doctrine that Protestants have issues with the Roman Catholic Church. So the early church functioned by a Trinitarian rule of faith until the Scriptures were written and collected together under on cover.
Check them out for yourself: Irenaeus, Against Heresies, 1:10:1; 1:22:1; 3:4:2; Tertullian, Prescription Against Heretics, 13:1-6; Against Praxeas 2:1-2, Athanasius, Letters concerning Holy Spirit to Serapion, 1:27, 28, 29. ( I want to read Origen’s first before I say anything about his, I will confirm that later.)
When the disputed books (only 7, 2 Peter, James, 2-3 John, Rev. , Heb, and Jude) were finally discerned and settled, they certainly laid down a principle that is very close to “sola Scriptura”.
When Athanasius wrote “alone” in his Festal letter 39, he was teaching an early church patistric principle, “in these alone are the fountains of salvation and godliness”, (Festal letter 39, the first listing of all the 27 NT books extant), he is expressing a form of Sola Scriptura that was later developed by Luther and Calvin. It is at least “material sufficiency” and comes really close to formal sufficiency. It was the early church principle of the early fathers that Luther and Calvin fully articulated “Sola Scriptura” from, because the Medieval church had added other doctrines and claimed that they were from the apostles.
Luther called the book of James, “a right strawy epistle”, only in comparison with Romans and Galatians and Ephesians and Paul’s doctrine of justification by faith alone, and he doubted it for a while, but he never removed it from the canon. And as for the word “alone” in Romans 3:28, several R.Catholic versions in German and Italian also had the word “alone” there, because the point of alone is not “without the presence of good works or without hte fruits and evidences of good works”, but “faith alone” means without the condition of meritorious works; that is without any works as conditions for justification.
So, Luther added nothing, and took away nothing.
As for the Deutero-canonicals, since Hippo and Carthage were local councils, they were never seen as authoritatively binding on all Christians until Trent, and they were always in debate until Trent formally proclaimed the deutero-canonicals as inspired by God. Jerome did not agree that they were canonical, and also Gregory the Great and Cardinal Cajetan also wrote that they were not canonical. It was Trent’s reaction to the Protestant Reformation that forced them to formally and dogmatically proclaim them “canonical” in a secondary sense.
