Spong's article includes a variety of helpful insights into the differences in birth narrative texts, and the interesting and surprising character of Matthew's genealogy. There are, however, a variety of assumptions and assertions in this article that deserve to be challenged--not the least of which is a rather amazing argument from silence.
At the heart of Spong's argument is the notion that the earliest New Testament writers, including Paul and the earliest evangelist, Mark, know nothing about the idea of the virgin birth, and that therefore the idea arose late in the New Testament period, perhaps on the basis of myths and legends about other famous births. Spong's conclusion is drawn from the fact that they write nothing about it. Spong also assumes that what they do write about Jesus' birth assumes there was nothing very special about it.
It is always a risky business to assume someone doesn't know something just because they don't mention it in the limited sampling of their writings that we now, many centuries later, have access to. In the first place, neither Paul nor Mark give a narrative account of Jesus' conception or birth. Had Mark offered us a birth narrative and written something about Jesus' origins that contradicted Matthew and Luke, that would be one thing. Then we could actually talk about differing accounts of Jesus' origins. This we simply do not have.
Paul simply says in passing in Galatians 4 that Jesus was born of woman. He does not say how this transpired, and in any case the issue of a "miracle" in regard to Jesus' origins has to do with the conception of Jesus, not the birth.
The birth of Jesus, according to what information we have from Matthew and Luke, was perfectly normal and ordinary. Indeed, it was so ordinary that, according to Luke, Mary went to Jerusalem and performed the normal purification ritual. Comments about Jesus' birth neither state nor necessarily imply anything about the conception of Jesus in Mary's womb.
It is, in fact, possible that Mark does bear witness indirectly to the scandal caused by the virginal conception; in Mark 6, Jesus is called "son of Mary" rather than "son of Joseph" by his hometown folks. This may suggest that they knew something peculiar had happened in regard to Jesus' origins--namely, that Joseph was not the father. An unfriendly interpretation of these origins would suggest Jesus was illegitimate. But we see a positive interpretation of those origins in Matthew and Luke.
Spong also trots out the tired old argument that there were plenty of virgin-birth legends out there in the first-century world, on which the evangelists could have modeled their narrative. This is simply false. The stories about the birth of the emperors (e.g., Augustus or later emperors), do not involve the idea of a virginal conception. What they often do entail is a narrative about how a god took on human form and had sex with a human female.
This is clearly not what the birth story in Matthew and Luke are about. Both of those stories are careful to stress that what happened was a miraculous conception in Mary's womb, not a mating between God and a human being. There are, in fact, no apt parallels in either Jewish or Greco-Roman sources to the stories in Matthew and Luke about the virginal conception.
Several other key points argue against the essence of Spong's interpretation of the virginal conception stories. First, he is partially right when he says the Hebrew term used in Isaiah 6 is not a technical term for "virgin." This is true--it means a young woman of marriageable age. What Spong fails to add is that in Jewish culture, this term would always imply the virginity of the person in question, unless the context suggested otherwise. In other words, while the Hebrew term does not focus on the virginity of the woman in question, the idea is nonetheless contained in the term, as is shown by the translation in the Greek Old Testament, which uses the term parthenos. This latter term does, indeed, focus more clearly on the virginity of the person in question.
I want to add that I quite agree that the story of Jesus' virginal conception is not likely to have been invented on the basis of the Isaiah text--when this text says "a young woman will conceive and give birth to a child," it does not go on to state how this will happen. Early Jewish readings of this text did not take it to refer to a miraculous conception without aid of a man's action. This means that the stories we find in Matthew and Luke referring to the virginal conception were neither derived from the Old Testament nor from Greco-Roman legends.
In fact, I would argue that it is highly unlikely Christians would make up a story about a virginal conception, precisely because it would lead to the charge of Jesus' illegitimacy by opponents of the Christian movement. There must have been some historical substance to this tradition for both Matthew and Luke to refer to the matter, independently of each other and in differing ways.
Evangelistic religions, like early Christianity, grounded in the life of a historical figure, Jesus, were unlikely to make up stories about their hero that would leave them wide open to the charge that Jesus was the offspring of an unholy union of man and woman.
As for the complaint that the gospel of John also does not mention Jesus' virginal conception, once again we are dealing with an argument from silence. Besides overlooking the fact that the prologue to John's Gospel is a hymn fragment--not a narrative account of the conception of Jesus--Spong also does not seem to realize that the concept of incarnation in that hymn and the concept of a virginal conception in Matthew and Luke are perfectly compatible notions.
The virginal conception focuses on how Jesus came into this world. The incarnation stresses that the Son of God in his divine nature existed in heaven before he took on flesh in Mary's womb, or as John puts it: "the Word took on flesh and tabernacled amongst us." Nothing in John 1 or elsewhere in this gospel contradicts the other birth stories. There are, therefore, not alternate mutually contradictory versions of Jesus' origins. Indeed, we have the testimony of two different and independent witnesses, Matthew and Luke, that there was indeed a virginal conception.
As the late Gospel scholar Raymond Brown argued in "The Birth of the Messiah," this dual and independent testimony to the virginal conception strongly suggests a firm historical basis for the story. Mary told someone about what had happened, clearing up questions about Joseph's lack of paternity.
We may be thankful that she did so.