If someone finds another letter written by Paul, I certainly would want to read it. However, to say that it must be read as authoritative is not only to have a wrong view about inspiration, but a deficient idea of God’s providence, and certainly a misguided belief about canonical authority.
I believe this question uses the word “authoritative” as meaning “inspired,” for I can not think of other meaning that would make sense in this context. Give this explanation I answer the question basing myself in this assumption to be true.
If we were to believe that letter to be inspired, we would be putting in checkmate our view of God’s providence and sovereignty. Where is God’s providence to provide his word to all the peoples that lived before the discovery of this letter? Was it not necessary by then? So God’s words contained in it are temporal? That presents a big problem for Jesus, since he said that God’s words “shall not pass away” (Mark 13:31). How can one sustain a strong view of sovereignty (I am not implying the philosophical categorization of different types of view regarding sovereignty) when his sovereignty was asleep during some centuries?
Back to the matter of inspiration, Paul himself wrote to Timothy that “all Scripture is inspired by God and is profitable for teaching, for rebuking, for correcting, for training in righteousness, so that the man of God may be complete, equipped for every good work” (1 Timothy 3:16-17). If that letter was not used by the church for more than 20 centuries, how could it be profitable for teaching, rebuking, correcting, and training? Could a “dead letter” be used to form complete men of God? Is it possible that a letter that was not used for more than 767.009 days be used to equip men of God to the good work? To say that it became authoritative only when they discovered the letter is to fall into a kind of “literal adoptionism.”
Paul was most definitely an Apostle of Christ Jesus, and I would not defy a man who had four personal encounters with Jesus himself (Acts 9; 18:9-10; 22:17; 23:11). Notwithstanding, I would be highly discouraged to accept all of his words as authoritative over the Church, for it was not he who was inspired, but some of his words. To say that these words in that letter are also authoritative one must answer all the questions mentioned above in a theological, philosophical, and ethical way that does not diminishes the image of God that orthodox Protestants lift up high.
I do not affirm that the letter should be considered as heretical rubbish, but that common sense and good theology must be part of the evaluation of this letter. Some might say: “but what if it cites Scripture? What if it cites other parts of Scripture?” And the answer continues to be, what about the sovereignty of God? Could not He have provided this before? Yes, he could, and as orthodox Protestant Evangelicals we believe he did, that is why we have 66 books, not 67. Justice Antonin Scalia said to Fox News, “The Constitution is not a living organism. It’s a legal document, and it says what it says and doesn’t what it doesn’t say.” Although Scripture is indeed a living organism, it could be regarded as being, not becoming. Scripture can not be improved because it was “once for all delivered to the saints” (Jude 1:3), and to say otherwise is to assume internal contradictions.