Isaiah's prophecy seems harsh. Ahaz loyally refuses to ask God for a sign. He says that he doesn't want to put the Lord to the test - a sentiment later echoed by none other than Jesus himself! But it would appear that, on this occasion at least, it's the wrong answer to give! The Prophet tells him that he should have asked for a sign, after all, and now he will be given one whether he likes it or not.
What's going on here? Perhaps Isaiah realizes that the real reason why Ahaz didn't ask for a sign is that he already suspects it will be inauspicious. Is this the royal equivalent of putting your hands over your ears and singing 'La, la, la!' to drown out the sound of bad news?
If so, the King is showing remarkable faithlessness because, in fact, the sign is not going to be the bad news he dreads. Instead, the sign is going to be full of hope. And what could be more hopeful than new life? Within two years - in other words, in the time that it takes for a woman to carry a child through pregnancy, wean him and begin to teach him the difference between right and wrong - the two enemy nations which are currently threatening Judah will have been turned into a desert. This is a true sign that God is taking care of the nation of Judah, and that is why the child shall be named 'Immanuel'.
There is, however, a scorpion's sting in the tail. The conclusion of the prophecy, which is not part of our reading this week, warns that Assyria - the great power which will soon destroy Judah's enemies - will bring upon Judah an even greater day of reckoning. It seems that Ahaz may not have been so stupid after all in choosing not to ask for a sign. He had actually asked Assyria to come to his rescue. In the short term that will seem like a smart move. But in the longer term Ahaz will learn that it would have been wiser if he had shown real trust in the Lord.
And what about the way that Matthew recycles the prophecy to explain the story of the virgin birth of Jesus? One problem with this borrowing of Isaiah's prophecy about Immanuel is that the mother who was expecting a baby was not actually a virgin, but just an ordinary young woman - possibly the wife of Isaiah or of Ahaz himself. The other problem is that Isaiah was not looking far into the future. He was simply explaining how something was going to happen very soon that would change the local situation out of all recognition.
Matthew, by comparison, is using the prophecy to show how the birth of a baby can have a far greater impact even than Isaiah imagined. God with us in Jesus will transform the entire course of human history. Now that really is a miracle! And that's why Matthew feels able to link the story of Jesus with the prophecy in another way. It's clear that there was already a tradition circulating that Mary had a virgin birth. What could be more fitting, thinks Matthew, for a baby who is destined to become God with us for all time?
Paul agrees with Matthew that the Good News of Jesus was promised beforehand by the Old Testament prophets, but there they diverge because Paul does not know the tradition of the virgin birth. Whereas Matthew describes how Jesus became a descendant of David by adoption, when Joseph married the already pregnant Mary, Paul tells us instead that Jesus was a physical descendant of David. For Paul, this means that Jesus is Son of God only in a spiritual sense, because the spirit of holiness dwelt within him, and not because he was conceived through the Holy Spirit's intervention.
It is one of the great conundrums of the New Testament that these two conflicting traditions co-exist side by side. Both traditions remind us of important aspects of the Jesus story. Jesus' descent from David is a reminder that he is the Christ, or Messiah, God's anointed or chosen leader - an aspect of the Gospel which is missing from the Isaiah prophecy. But the story of God With Us in a tiny baby adds another dimension to the messiahship of Jesus. He is not only someone sent by God to save us, he is also God come amongst us to share our human experience. And although they may have started from different historical traditions about Jesus, Matthew and Paul are agreed that both these aspects of the story are vital to the Good News they have to proclaim.
Bishop David Jenkins, the former theology professor who loved to shock journalists with his unexpected statements about the Christian faith, said this about the Christmas story: "Christmas confronts us with a baby as the glory of God. The real wonder did not, and does not, lie with angels and shepherds or a guiding star from the East. All these are derived wonders. They only point to the true wonder. They symbolise the faith and reflect the glory.
"The real glory, the lasting glory and the undeniable glory is the baby, who grew up as Jesus of Nazareth to be 'crucified, dead and buried'. But this was the beginning rather than the end: for the God, who he named with particular passion, raised Jesus up. So Jesus was known to be Christ the Lord, the power of God's kingdom, the means of judgement and the promise of God's future. Thus when we celebrate the birth of Jesus Christ we are confronted with a baby as the glory of God."