If asked what Christmas is about most Christians would, I hope, agree that what Christians celebrate at Christmas is the birth of Jesus, the Son of God. However, in my experience what most Christians don't stop and think about is what exactly the Bible means when it calls Jesus the 'Son of God.'

In this article I want to explore how the Bible describes Jesus as Son of God in two different ways, as the king descended from the line of David, and as the second person of the Trinity, the eternal Son of God the Father.

A good way to start thinking about these two uses of the term 'Son' is to look at Luke's account of the announcement by the angel Gabriel to Mary that she is going to give birth to Jesus. In this story we find the following familiar words which we hear year by year in church services at Christmas time:

"And the angel said to her, 'Do not be afraid, Mary, for you have found favour with God. And behold, you will conceive in your womb and bear a son, and you shall call his name Jesus.

He will be great, and will be called the Son of the Most High;
and the Lord God will give to him the throne of his father David,
and he will reign over the house of Jacob for ever;
and of his kingdom there will be no end.'

And Mary said to the angel, 'How shall this be, since I have no husband?' And the angel said to her,

'The Holy Spirit will come upon you,
and the power of the Most High will overshadow you;
therefore the child to be born will be called holy,
the Son of God' (Luke 1:30-35)."

Many Christians who hear these words read at Christmas services assume that when Gabriel refers to Jesus as 'the Son of the Most High' and 'the Son of God,' he is making a reference to Jesus' divine nature. This way of understanding these words is perfectly understandable given the way in which down the centuries Christian theology has used the words 'Son' to refer to the second person of the Trinity as a way of distinguishing him from the first person of the Trinity, God the Father.

Thus, Article II of Church of England's Thirty Nine Articles refers to:

"The Son, which is the Word of the Father, begotten from everlasting of the Father, the very and eternal God, and of one substance from the Father, took Man's nature in the womb of the blessed Virgin, of her substance."

Here the word 'Son' is a description of Jesus as the second person of the Trinity, the Son who is eternally begotten from the Father.

However, this is not what the term 'Son' means in Luke 1:30-35. Rather, in Luke the term refers back to the use of the word 'son' in the promise made by God to King David back in 2 Samuel 7:12-14 of a human descendent of David to whom God will grant an eternal kingdom:

"I will raise up your offspring after you, who shall come forth from your body, and I will establish his kingdom. He shall build a house for my name, and I will establish the throne of his kingdom for ever.I will be his father, and he shall be my son."

If we ask when Jesus becomes God's son in fulfilment of this promise, the answer that Luke subsequently gives us is that Jesus is set upon the throne of his father David when he is seated at the right hand of God following his resurrection and ascension. This is what Peter means when he says in his sermon on the Day of Pentecost 'that God has made him both Lord and Christ, this Jesus whom you crucified' (Acts 2:38).

In the same way Paul says in Romans 1:4 that Jesus was 'designated Son of God in power according to the Spirit of holiness by his resurrection from the dead.'

In these two passages 'Lord,' 'Christ,' and 'Son of God' mean the same thing, the king descended from David who rules the world on God's behalf, and Jesus becomes this king as a result of his resurrection.

When I point this out, people generally say to me 'Does this mean that Jesus was not Son of God from all eternity?' The answer to this question is 'Not at all!' This is because what the New Testament tells us is that the very same person who becomes son as the king descended from David has also always been God's Son as the second person of the Trinity.

The New Testament witnesses to this truth in two ways.

First, those writers who describe Jesus as the king who ascended to the throne of David after his resurrection are also clear that Jesus is none other than God himself.

We can see this in Luke's account of the nativity, for instance, when Elizabeth refers to Mary as 'the mother of my Lord' (Luke 1:44), 'Lord' here referring to YHWH, the Lord God of Israel. Mary, says Elizabeth, is the mother of none other than God himself. We also see it in Romans 9:5 where Paul declares that Jesus is the Christ (i.e. the promised descendant of David) but he is also 'God over all blessed for ever.'

Secondly, the New Testament uses the term 'Son' to refer to Jesus' divine nature as well as to his status as the promised Davidic king.

Thus, in Hebrews 1:1-3 we read:

"In many and various ways God spoke of old to our fathers by the prophets; but in these last days he has spoken to us by a Son, whom he appointed the heir of all things, through whom also he created the world. He reflects the glory of God and bears the very stamp of his nature, upholding the universe by his word of power."

Here Jesus as Son shares his Father's divine nature (which is what 'bears the very stamp of his nature' means) and for this reason he reflects God's glory and shares God's unique role of creating and ruling all things.

In similar fashion, in the prologue to John's gospel 'the only Son from the Father' whose glory we have beheld (1:14) is none other than the Word who from all eternity 'was with God' and 'was God' (John 1:1). He is 'God the only begotten' (John 1:18).

In the words of the New Testament scholar Thomas Schreiner, what we learn from the New Testament is therefore that, 'The one who existed eternally as the Son was appointed the Son of God in power as the Son of David.'

To put it another way, he who is already Son eternally through his divine nature also becomes Son in the Davidic sense through the successful completion of his incarnate mission.

And all of this is good news. It is good news because only the eternal Son, the second person of the Trinity, possesses the divine power necessary to save us by destroying sin and death forever, and the way he does this is by being born from the Virgin Mary as a human being at the first Christmas and then suffering, dying, rising, ascending and ruling on our behalf.

As Robert Jamieson puts it in his book The Paradox of Sonship:

"Christ's divine and human constitution and his faithful execution of his incarnate mission are integral to his ability to save. Only one who is divine; who became human; who endured temptation and gave his life in death; who was raised incorruptible; and who now reigns in heaven can deal decisively with sin, give us access to God, and make the new creation our permanent possession."

Martin Davie is a lay Anglican theologian and Associate Tutor in Doctrine at Wycliffe Hall, Oxford.