What is central is that Spong apparently does not know what 'midrash' actually is. The 'genre' of writing to which he makes such confident appeal is nothing at all like he says it is. There is such a thing as 'midrash'; scholars have been studying it, discussing it, and analysing it, for years. Spong seems to be unaware of the most basic results of this study. He has grabbed the word out of the air, much as Barbara Thiering grabbed the idea of 'pesher' exegesis, and to much the same effect. He misunderstands the method itself, and uses this bent tool to make the gospels mean what he wants instead of what they say.
We may briefly indicate the ways in which genuine 'midrash' differs drastically from anything that we find in the gospels.
First, midrash proper consists of a commentary on an actual biblical text. It is not simply a fanciful retelling, but a careful discussion in which the original text itself remains clearly in focus. It is obvious that the gospels do not read in any way like this.
Second, real midrash is 'tightly controlled and argued'. This is in direct opposition to Spong's idea of it, according to which (p. 184) 'once you enter the midrash tradition, the imagination is freed to roam and to speculate'. This statement tells us a good deal about Spong's own method of doing history, and nothing whatever about midrash. The use made of the Old Testament in the early chapters of Luke, to take an example, is certainly not midrash; neither is it roaming or speculative imagination.
Third, real midrash is a commentary precisely on Scripture. Goulder's theories, on which Spong professes to rely quite closely, suggest that Luke and Matthew were providing midrash on Mark. It is, however, fantastically unlikely that either of them would apply to Mark a technique developed for commenting on ancient Scripture.
Fourth, midrash never included the invention of stories which were clearly seen as non-literal in intent, and merely designed to evoke awe and wonder. It was no part of Jewish midrash, or any other Jewish writing-genre in the first century, to invent all kinds of new episodes about recent history in order to advance the claim that the Scriptures had been fulfilled. It is one of the salient characteristics of Jewish literature throughout the New Testament period that, even though novelistic elements could creep in to books like Jubilees, the basic emphasis remains on that which happened within history.
A moment in the debate that particularly struck me was when Spong related how Carl Sagan had once approached him and said something to the effect of, if Jesus had ascended away from the surface of the earth at the speed of light, he'd still be in the Milky Way galaxy. This is essentially the claim that the Ascension was predicated on a local heaven just above the clouds and thus that the ancients and medievals didn't know the universe is incomprehensibly large, something I showed to be false here.
The fact that Spong thinks this is a good or original point further demonstrates how insulated he is. The South England Legendary, written in the 13th century, says something similar. C. S. Lewis writes in The Discarded Image, that the Legendary is "better evidence than any learned production could be for the Model as it existed in the imagination of ordinary people. We are there told that if a man could travel upwards at the rate of ‘forty mile and yet som del mo’ a day, he still would not have reached the Stellatum (‘the highest heven that ye alday seeth’) in 8000 years." Since this was common knowledge several hundred years before it occurred to Sagan or Spong, I can't get too excited about their "insight", much less their claim that it threatens traditional Christianity -- a point that the South England Legendary somehow misses.
(cross-posted at Quodlibeta)