A problem for Protestants seeking to craft criteria for choosing which books belong in the canon of Scripture is the fact that, even if we had irrefutable evidence that an Apostle wrote a particular letter, that doesn’t necessarily mean that God had inspired it.
To add another obstacle, we do not have irrefutable, or at times even compelling, historical evidence that an Apostle wrote a particular book. Take the letter which we know as St. Paul’s letter to the Ephesians: firstly, we don’t know that St. Paul wrote it and secondly, we don’t know whether it was even written to the Ephesians! Here’s what the Navarre Study Bible says in its preface to the letter:
Study of the literary features of the letter does not enable us to say with certainty whether it was written by St. Paul himself or (and this is what many scholars nowadays think more likely) by a successor of his who holds in high regard the “holy apostles and prophets” (see 3:5) and who, inspired by the Holy Spirit, wrote the letter in order to shed further light on the faith held by people who owed their conversion to apostolic preaching.
Certainly, Tradition has always regarded the letter as having been written by St. Paul himself. The Church has taken it to be a book of sacred writing and has, from the start, included it in the canon of Holy Scripture.
But since a Protestant does not have certainty that the Church in the early centuries was listening to God correctly in making decisions on doctrine (including the canon), a Protestant cannot have certainty from the sacred Tradition of the Church that she made the right decision on Ephesians. Yet, a Protestant also cannot have certainty by relying on literary analysis (which is ambiguous in any case) or by the small sampling of early Christians who claimed that St. Paul did write this letter. He’s caught in a pickle.
The pickle is made even more sour by the fact that it seems probable from the Bible that St. Paul wrote other letters not included in the Bible. And most likely the other Apostles did, too. So just because an Apostle authored it, does not make a letter inspired. Further, someone like St. Luke, who was not one of the Apostles, wrote a tremendous portion of the New Testament.
Analysis of historical testimony and literary features can never give us conscience-binding certainty that a book is inspired by God. Instead, we must have faith that God protected the early Church from error in her discernment of the canon. If God didn’t protect this discernment, then we cannot have certainy that the canon chosen was correct. Protestants do not believe that God protected the Church from error, even in the early four centuries when the canon was chosen, so they cannot have conscience-binding certainty for their canon. Catholics believe that God promised to lead his Church “into all truth” and that he has followed through on his promise by protecting the Church from error in her teachings on the faith, which includes the discernment and teaching of what the canon of Scripture is.