Last week, St Matthew-in-the-City posted an ill-considered and, to many, insensitive billboard. The intention, so Archdeacon Glynn Cardy is reported to have said, was to spark debate about the true origins of Christmas. The problem was the matter for debate was wholly ambiguous.

Like others, I found the billboard objectionable. This was not because it depicted Joseph and Mary in bed together. Nor was it the suggestion that they had just had sex.

Despite what some argue about Christian tradition maintaining that Mary remains a virgin, this is difficult to substantiate on the evidence of the New Testament.

Matthew's Gospel merely states that Joseph abstained from sexual relations with Mary until after Jesus was born. All the gospels report that Jesus had brothers (Mark and Matthew). The contexts of these references suggest that they were natural brothers. Later church tradition has suggested that they were cousins, or half-siblings. The tradition of Mary's perpetual virginity dates from the second century.

The idea is first found in a book called The Protevangelium of James, which the church did not consider authoritative.

My difficulty with the billboard lay in the slogan that "God was a hard act to follow". What was this supposed to imply? Was it meant to suggest that, after the honour of supernaturally conceiving Jesus, the Son of God, any subsequent natural conception was bound to be a let-down? The gospels of Matthew and Luke say that the conception of Jesus was the result of the power of the Holy Spirit. It is difficult to know what St Paul thought about "the virgin birth" as he simply notes that Jesus was "born of a woman". But there is no doubt that he understood that Jesus was God, and that by an initiative of God, "the Son" became human.

This is really what the virgin birth is conveying. It signifies the understanding that in Jesus, God became a human being. And this happened in a real and historically situated way. It is not simply a metaphor, or a fiction. This astounding reality is what Christians celebrate at Christmas: that God has shared our human life. And it was not a nice, comfortable middle-class life.

Think of it: the scandal of a pregnancy out of wedlock, a birth in transient circumstances, the family of Jesus refugees from a paranoid ruler quite happy to unleash "state terrorism" on unsuspecting villagers. Luke says that the news of the birth was told first to shepherds, often considered to be thieves, and not religiously upright or observant. Matthew says that early visitors were foreign astrologers bearing symbolic gifts. Early on in Jesus' life, his mother Mary was told that a "sword" (of sorrow) would pierce her heart.

God became human in Jesus in order to engage in a mission of reconciliation to overcome the alienation that had come through human waywardness. John's Gospel speaks of the coming of the Word in order to give people "the right to be children of God".

This was to come about on account of the death of Christ. This is why many Christians celebrate Christmas by remembering the death of Jesus.

Unfortunately, as an exercise in getting a conversation about the original meaning of Christmas going, the St Matthew's billboard has generated more heat than light. Perhaps a billboard such as this might do the trick. A conventional (or contemporary, if you like) stable scene, with the words: "What on earth is God doing here?"

* The Rev Dr Derek Tovey is a lecturer on the New Testament at St John's College in Meadowbank, Auckland.