I wanted to post here a good discussion Dick Norton and I have been having on the polyvalence of the Old Testament for some time. He is taking the position that the Old Testament passages had double meanings but that they were understood at the time of writing.
There are at least two major problems with your polyvalence theory in my estimation.
First, to use Isaiah 7 as an example, the prophet Isaiah himself saw the coming of Messiah foreshadowed in his original prophecy to Ahaz, for Isa. 7:14 is in an expanded context that ends with Isa. 9:6-7. Of course, Isa. 7:14 was meant for Ahaz, but as Isaiah contemplated the near term fulfillment of that prophecy through the birth of his own son, he also began to see that the birth of his son was a foreshadowing of the birth of a greater Son to come in the future. Therefore, it is altogether appropriate for Matthew to see the fulfillment of Isa. 7:14 in Messiah's birth. He is simply following the greater context of Isaish's prophecy!
Second, if the N.T. writers and preachers made such cavalier use of the O.T. by seeing fulfillment of the O.T. prophecies in Jesus, why would they not have been laughed out of the synagogues by the Jewish scholars? In order to proclaim such an astounding message that Jesus of Nazareth was indeed the Messiah, they would have to use the O.T. in the way the O.T. interprets itself. Jewish scholars wouldn't have let them get by with such "spiritualizing" of the O.T. message! I have made both of these arguments with you before, and I don't remember your having answered either one of them.
My fear is, brother Ken, that you are using exegesis that is destructive to the true original meaning of the O.T. because you haven't grasped the true unity of God's word. Jesus is indeed the Messiah promised by the O.T. writers, looked forward to by the Jews of antiquity, anticipated by first century Jews such as Simeon, and openly revealed by Jesus himself as he expounded the O.T. Scriptures about himself on the Emmaus Road.
Dick, I do indeed believe that God knew these texts would have these resonances at the time of Christ and am even willing to believe he tweaked wording for them to do so (for example, the Septuagint translation of Isaiah 7:14). I just don't think it likely that Isaiah or the Israel of his day heard them that way. The original meaning question must always be, "What would this text have meant to its first audience?"
Our debate is not over whether these words can have Christological meanings, which I believe they legitimately can. Our debate is whether the original authors and audience intended those meanings. For example, I don't see how anyone can be called a competent original meaning interpreter who thinks Hosea was thinking of Jesus when he wrote 11:1 (or that anyone prior to Jesus thought of the verse that way).
My response to your second point is two fold. First, that you are being modernist even in your picture of Jewish scholars. The New Testament authors were using exactly the same methods that Jewish scholars of the time used (e.g., the interpretive canons of Hillel the Pharisee). That's why these methods cannot be considered in error. Only when a modern standard is applied can they be considered in error. In fact, many think that Paul's allegory of Sarah and Hagar was a counter-allegorical interpretation to an initial allegorical interpretation by his opposition.
But also, there was Jewish opposition to the NT author's use of the OT. It is often suggested that the translations of Symmachus and Aquila were produced specifically because the Septuagint was too easily used by Christian Jews to make their arguments.
As for Isaiah 9, I do wonder if God did a lot of nudging in the wording. But did you realize that no NT text applies Isaiah 9:6 to Jesus (Wonderful Counselor, Mighty God)? We're so used to Handel's Messiah that we might not realize the Bible never uses this verse of Jesus. The NT does use other verses in this chapter of Jesus (e.g., 9:2 in Matthew), but not the one that seems so naturally to be a prediction of Christ.
Nevertheless, the question for this post is whether Isaiah himself understood the verse that way. What would this text have meant to Israel at the time? There was already a king so would they not have read it in terms of a coming heir to the throne? Hezekiah, son of Ahaz, is usually suggested by original meaning scholars.
The exalted titles of 9:6 are similar to the kinds of divine titles Egyptian kings received, and Psalm 45:7 shows us that the human king of Judah could be figuratively called God (Psalm 45 was originally a wedding psalm) in lieu of him being God's earthly representative. So I am very willing to say that God formed the wording of Isaiah 9 to be ripe for the time of Christ. But there is nothing in this chapter that could not have been said, given the standard practices of the day, about Hezekiah or a human king.
And I believe that is how Isaiah understood the words. I don't know of any evidence of any Jew prior to Christ reading these words in the Christian way. In fact even the NT doesn't of the verse you most have in mind.
Again, I have no problem with seeing Christ in the Old Testament or with reading the biblical text theologically as a unity. The problem is with a reading that will not let the original Old Testament meaning speak also on its own terms. I believe it will lead many honest interpreters into a faith crisis.
Christ has died; Christ is risen; Christ will come again!