This post wasn't really pointed in my direction, but I think it gets right to the heart of most interreligious difficulties among the main Christian denominations.
What is at issue, then, is the elementary and elemental question of whether or not the Bible is an open book. This was foundational to the Protestant Reformation. Can we, by acquainting ourselves with as much as has survived of the civilizations in which God revealed the Bible, enjoy, to that degree, the same access to the meaning of Scripture as the original audience enjoyed?
Personally, I doubt it, and that's not a particular criticism of Scripture but of any attempt to reconstruct an ancient document. You can make a good run at it, but the degree of certainty one might have about the meaning is less than absolute, and there is likely going to be a massive amount of disagreement about how probable any particular person's conclusions are with regard to several important issues of interpretation. I think generally you can make pretty good guesses about meaning, but in lots of particular instances, you're probably going to have to throw up your hands and say "we just don't know for sure." It appears to me that the Magisterial Reformers recognized that and evaluated dubious cases (such as paedobaptism, rebaptism, and heretical interpretations) according to tradition, which they considered binding (even if not infallible) in areas where the Scriptural witness was unclear.
Put another way, wasn’t the Bible written to be understood? Doesn’t it mean whatever it was meant to mean for the implied reader and the target audience? Of does he deny the adequacy of human language as a vehicle of divine revelation?
False dichotomy. There's no necessary relationship between whether the Bible means ONLY what it was meant to mean for the implied reader and the target audience and whether human language is an adequate vehicle for divine revelation. The standard Christian account of revelation for centuries was that it was written by God for the Church collectively, not simply a fixed message between an author and a receiver at the time. The literal sense was always considered true, of course, but not limiting. The Bible was clearly written to be understood, but understood by whom and when is an entirely different question.
What supplies the interpretive grid? Original intent, synchronic with the historical audience and authorship or Scripture? Or post-biblical tradition, diachronic with church history?
If you're interested in the God-inspired meaning, both (at least according to the traditional account). If you neglect either, you cut off part of the intended meaning. If you adopt either extreme, it's hard to see how you haven't strayed into some kind of Christological error regarding how the divine and creation intersect. The former idea particularly seems to view the divine and human qualities of Scripture as separate, which is analogous to either Nestorianism or Arianism/Adoptionism (if it's simply God's use of human instruments). The latter view is analogous to Docetism, neglecting the historical reality of the Scripture.
To take a comparison, when we interpret Dante, do we limit ourselves to Dante’s past and present, to Dante’s 14C Florence and Dante’s education and Dante’s literary allusions, or do we interpret him in light of Hubble and Einstein?
If we knew for a fact that Dante knew the end from the beginning and providentially ordered things so that the significance of part of his writing would be realized centuries later, then sure, we might interpret him in light of Hubble and Einstein. That's the difference between the author being God and the author being Dante. :-)
Surely the question answers itself. We interpret Dante by what comes before, not what comes after. And the same historical threshold applies to Scripture, for Scripture is the record of historical revelation with a historical point of origin in the 2nd millennium BC and chronological cut-off in the 1C AD.
That defines the objective content of Scripture, but not what is received from Scripture (the subjective Christian belief that ought to be taken from it at a given time). In other words, asking the question "What would a first century Christian have taken this to mean?" doesn't exhaust the possible meaning that God intended. Exegesis gives the former, but only the witness of the Church through the ages gives the latter.