There is no better context to ponder the Incarnation than the Service of Nine Lessons and Carols (this year @ our Cathedral @ Christ's College Chapel). In my mental wandering I explored how John came to determine that 'the Word became flesh' given that little in the other gospels points directly to that conclusion, and, make what we will of the Matthean and Lukan birth narratives, it is not necessary to conclude that the virgin birth = divine Father, human mother = the Word became flesh.
Of course we can properly imagine that what John writes in 1:1-18 is revealed to him through the Spirit whom Jesus promises will lead the disciples into all truth. But revelation comes to human minds and needs to be received in order to be transmitted onwards. We may properly ask what was going on in John's mind that he readily wrote 1:1-18. Something lay there already, ready for the conviction that in Jesus Christ the Word became embodied in human flesh. What might that have been?
We know, reading further into John's Gospel, that he was convinced that Jesus Christ was 'Son of God' in some specific ways, most importantly for this exploration, 'Son of God' in a unity with God the Father. (We may recall that 'Son of God' could be used of Israel itself, but Israel and God in the first century, under the lordship of Rome, were not in a state which Israel could triumphantly acclaim as a unity). This unity between Father and Son is attested to in Matthew 11:25-27 and in Luke 10:21-22. Whether John knew these gospels as written documents or not, the gospel traditions apart from John attest to a conviction that Jesus Christ was the Son, and with the Father he had a relationship marked by intimacy and identity which spoke of unity in purpose and in will.
But if the Father and the Son were 'one', what did this mean for the overall course of the existence of the Son? Was this unity arrived at when the Holy Spirit baptised Jesus (Mark), or when Abraham obeyed God and set in train the genealogy of Israel (Matthew) or when Adam himself was created as the first human creature (Luke)? John pushes further and deeper into the mystery of the relationship between God the Father and Jesus Christ the Son. If Jesus speaks the words of God (the Johannine discourses), enacts the power of God (the signs), and represents himself as God with his people ("I am" declarations), is not God present in Israel in a manner new to Israel? In Jesus Christ the divine has broken the bounds of the Temple yet being present in such a way as not to destroy those in contact with the divine presence.
Where in the theology of Israel were the means of expressing this new development in God's relationship with Israel? How could God be present among his people in a direct way without destroying them?
Although not directly expounded in the Old Testament, many stories there are a theological engagement with a thorny philosophical problem: how can the divine (Infinite, Holy, Wholly Other) make connection with the human (Finite, Profane, Common)? God in God's Godness is an all consuming fire and cannot come near humanity if humanity is not to be destroyed. So the OT has stories of encounters with angels, with the minutest momentary sight of the glory of God, and develops talk about the Name of God, or the Glory of God and the like, in order to express real presence of God which is not so direct and raw that humanity cannot survive it.
But through the whole Old Testament there is one particular way in which God comes to Israel and spreads himself (so to speak) through the commonness and profaneness of Israel. This way is through the spoken words of God. How does Israel 'see' God and live? It hears what God says. Through the words of the prophets (including Moses) God comes near to his people; in receiving the words of God, the people enter into relationship with God without being destroyed. Indeed Israel comes to understand that the words of God are God's ways of acting in the world. Indeed the world itself comes into being from God by means of his words (or commands) being spoken. In the wisdom literature further insight is expressed: the words of God are collectively one personal word coming from God: Wisdom (Proverbs 8:22-36).
When John theologises he does so in a context which has received some further insight. Wisdom coming from God not only created the world, it sustains the world as a core principle of its order, the Logos of Greek philosophy. So the stage is set for John to set forth the 'divine drama' in a fuller script than expressed in Matthew, Luke and Mark. In this fullness of disclosure through the Spirit, Jesus Christ the Son is one with God the Father because it has always been so, from before the beginning of time. (Indeed it would be less than oneness if there was a time when there was not unity between them).
But what kind of God (the Father united with the Son) can enter human existence and not destroy it? The simplest, safest, and, with respect to the OT, most sensible way for this to take place is for the Wisdom/Logos of God to take on human flesh. The Word of God as a speaking to humanity has already been the means for God to connect with humanity, but the speaking has been through prophets as 'servants' of God. Now the speech of God comes through one who is more than a servant, but shares with the prophets their common humanity.
The Word, so to speak, needed an appropriate or 'becoming' means to enter into human life in a new way. Flesh became the Word when the Word became flesh.
At Christmas time, in services such as Nine Lessons and Carols, the high point of all possible relevant readings is John 1:1-14. Rightly so. This is testimony to the true newness of God's work in the world, as well as to new truth of God, yet built on a foundation in the Scripture of Israel: God is with us, the Word has become flesh.