Rome insists that she is an authentic interpreter of Scripture. We can easily provide an example, within a document defining a dogma, of Rome making a clear blunder. But let's leave that aside for a second, and consider the effect of Rome's claims on a conversation.
Christian: We should reject Marian devotion because the Bible teaches us to trust in God alone.
Roman apologist: You have wrongly interpreted the Bible. Only Rome can authentically interpret the Bible.
Christian: That's not true, the Bible was written to be understood. Anyone can authentically interpret the Bible, and many do - some more, and others less, well than others.
Roman Apologist: No, you cannot understand the Bible without the Roman Catholic church.
Christian: That's not so.
Roman Apologist: Look, it says so right here in Matthew 16:18.
Now, that appeal to Scripture looks an awful lot like the Roman Apologist conceding that people can understand the Bible without the Roman communion. But behind that appearance lies a question about what this Roman hermeneutic entails.
1) Is it like special decoder glasses?
Is the Bible simply incomprehensible on its own, and one needs the Roman church to provide spectacles to make the incomprehensible, comprehensible? If that were true, then it would make no sense to appeal to Scripture to anyone not already looking through the spectacles.
2) Is it like the answer key to a Rubik's cube?
Is the Bible simply highly complicated, and one needs the Roman church to show the map of the way through to get the solution? If this were the case, the appeal to Scripture might make sense. This is just the first breadcrumb along a trail that eventually leads to Rome. In fact, though, all of Rome's attempts to prove her distinctive doctrines from Scripture fail. When you get an answer key to a Rubik's cube, you can see the parts all come together to form the solved puzzle, even if you couldn't have done it on your own. But with Rome, you don't get satisfactory answers like that. You get alleged solutions, but even knowing the supposed solutions, one cannot arrive at these solutions from Scripture.
3) Is it like the person who showed you how to look at "Magic Eye" 3D pictures?
Sure, at first it was just a weird bunch of lines and patterns, but once you were taught how to change your focus, suddenly the beautiful stereoscopic patterns emerged. Some of Rome's converts stories make it sound like they feel Rome's hermeneutic is similar to this. The two problems are - first, they don't seem to be able to teach us how to see the butterfly amidst the squiggly lines - and second, until we see the butterfly, appeals to Scripture are just appeals to squiggle lines, and consequently futile.
4) Is it like Humpty Dumpty?
In Alice Through the Looking Glass, she encounters the character Humpty Dumpty who insists on making words mean what he wants them to mean, even when that meaning is quite distant from any conventional sense of the word. Some of the arguments from the Roman side favor this interpretation. After all, some Roman apologists try to approach the Bible as though it were the creation of the Church, rather than being God's word delivered to the churches (and CCC 111 and 113 seem to encourage them to take this approach). If the Bible were the product of the Church, then the authorial intent behind the words becomes important, and we need to let Humpty Dumpty use words like "only mediator" in a far from conventional sense. One problem with that is that it turns the text of Scripture into such a "living document" that the document itself has no particular significance. Matthew 16:18 might as well teach the papacy as it teaches the bodily assumption of Mary, so long as Rome says that is what it means. The fact that we don't see it in the actual meaning of the words doesn't matter.
Ultimately, no matter what we liken the Roman hermeneutic to, we should realize that the Roman hermeneutic boils down to sola ecclesia: what Rome says goes. If the Bible appears to say the same thing, and that convinces someone that Rome is right - great. If the Bible appears to say the opposite, the Bible's apparent meaning should be subordinated to what Rome teaches.
But if that's Rome's hermeneutic, then the appeals to Scripture as an authority are really disingenuous. Honest Roman apologists shouldn't argue that we should believe them because (to use their lingo) we interpret the Bible the same way they do. After all, when we interpret the Bible differently, we're supposed to just set that aside, no matter how clear the Bible is.
Yet, I welcome comments from Roman apologists, clergy, and even laity. To what do you liken the Roman hermeneutic, and to what shall I compare it? And when you try to quote the Bible to me, do you think I'm just unaware that your church teaches that "all that has been said about the manner of interpreting Scripture is
ultimately subject to the judgement of the Church which exercises the divinely
conferred commission and ministry of watching over and interpreting the Word of
God" (CCC 119, quoting Dei Verbum 12, 3rd paragraph)?
P.S. Oh, and by the way - the alternative is that the Bible is the very word of God, and that God made it clear enough to serve as a rule of faith and life for his church. Not all parts are equally clear, however, and sin blinds the minds of some men so that even the most clear parts become dull. Nevertheless, core doctrines (like the contents of the Apostles' creed, for example) are plainly and unmistakeably set forth in the Scriptures, without the need for any special glasses, tricky eye techniques, or authoritative lexicography.