Doug Chaplin is pondering a few commonalities, including “a clear verbal parallel at the core,” between Matthew and Luke’s versions of the story of Jesus’ birth. What if Matthew didn’t “invent” his story of Mary’s virginal conception in order to fit his messianic interpretation of Isaiah 7:14 but rather appealed to Isaiah 7:14 to explain a detail he already knew from his Jesus tradition?
Matthew is certainly keen on demonstrating Jesus as the fulfilment of prophecy, and capable of moving backwards and forwards between prophecy and the Jesus story. We must therefore weigh each instance to see whether it is more likely that Matthew has come up with a detail which (absent any real knowledge) must have happened in this way because of what the prophet said, or whether he has decided this or that verse is a prophecy because it fits with the Jesus tradition as he has received it.
It is certainly plausible, for example, to think that either Matthew or his tradition come up with the birthplace in Bethlehem because of Micah’s prophecy (though I’m not fully persuaded). It seems to me a good deal less plausible that a virgin birth should be invented on the basis of a verse no-one else ever seems to have taken as Messianic. I would be more inclined to think that this is Matthew “scripturizing” (a term I owe to Mark Goodacre) the tradition he has received, and justifying the rather odd new Christian belief about Messiah for his somewhat beleaguered Judeo-Christian community.
If I’m right, that Matthew is innovative in offering this rather odd reading of Isaiah 7:14 because of a prior belief about Jesus, perhaps it may also be a further reason for thinking he is working with a pre-existing tradition about the birth of Jesus, and indeed, one he shares with Luke.