When a Protestant is asked for a Bible passage that proves that the Bible alone is the only authoritative guide in Christian faith and practice (and there needs to be one or else the idea is falsified at the outset) the one that is typically appealed to is this one:
2 Timothy 3:16-17 (ESV) 16 All Scripture is breathed out by God and profitable for teaching, for reproof, for correction, and for training in righteousness, 17 that the man of God may be competent, equipped for every good work.
2 Timothy 3:16-17 (NASB95) 16 All Scripture is inspired by God and profitable for teaching, for reproof, for correction, for training in righteousness; 17 so that the man of God may be adequate, equipped for every good work.
Now it seems to me that debate about proper interpretation of this passage is an asymmetric one in one sense. In the 1500s the Protestants made the startling claim (well, it startled many in the church) that the Bible was the sole and sufficient guide for the church; that Bible interpretations of bishops, popes and councils were not authoritative. They sought to overthrow the accepted view of church authority with their own. On this basis they divided the church, despite Christ praying that His followers would be one.
So it seems to me that the burden of proof on the Protestants, when it comes to these verses, is not whether they might possibly be interpretated as supporting their position, but that they lead inescapably to that conclusion. After all, sola scriptura is a mere meaningless abstraction without the complementary doctrine of the perspicuity or clearness of scripture, i.e. that the Bible is clear in its meaning in at least the important parts. (Who gets to decide which parts are inportant or which doctrines are basic or foundational is another question.) On the other hand, if these verses might or might not be interpreted that way then the preexisting Catholic position is not falsified and the Reformers had only an uncertain basis upon which to judge their own teachings right and Rome’s as wrong. (Applied more generally, if your interpretations of the infallible Book are not themselves infallible, then that still leaves you with uncertainty.)
So do the verses lead inescapably to the view that the Bible is the only authority in the church? I don’t see that. For one thing, we don’t see the word “only” which makes a huge difference. They say that scripture was given so that the man of God might be competent or adequate (Gr. artios, well-equipped). But this is not the same logical construction as saying that by scripture alone the man of God is adequate. Consider this analogy: A mechanic needs a variety of tools to do his work: various wrenches, sockets, meters, hammers, torches, etc. etc. Say a mechanic had most of the necessary tools but lacked a socket set. He would not be adequately equipped. Suppose you then gave him a socket set so that he would be adeqately equipped. You would not then turn around and conclude that with a socket set alone a mechanic would be equipped. In the same way, scripture is one of the necessary components for a well-equipped man of God. Otherwise the verses are pressed too far and “prove,” for example, that the illumination of the Holy Spirit is not necessary. Seminary and original language training are not necessary. Classes in hermeneutics are not necessary. But most sola scriptura-believing Protestants would not say that. A handful do, and they are usually the ones handling snakes in ramshackle Appalachian churches. We can’t have it both ways. Either these verses rule out the need for all these things, or if not then they also fail to rule out the need for sacred Tradition handed down from the apostles to aid in properly understanding scripture .