It is significant that the second person of the Trinity is called the Son. Our automatic translation of "Son" into "Jesus" is a mistake that excludes the witness of the Hebrew Scriptures.
The continuity between the writers of the Old and New Testaments is that the history of the nation Israel and the history of the man Jesus were central to their concerns i.e. their understanding of the reality of God was based on human experience in time. This means that Israel was unique among the nations, not simply because they wrote their history, the Romans also did that, but that they interrogated their history in order shed light on the relationship between the nature of the world and their own nature, to inform us of the troubles and delights of the flesh.
The writers of the Hebrew Scriptures used their experience of human suffering, the contingency of all things, the loss of temple and land and the apparent absence of God, to write extra-historical books like the book of Job, Ruth, Jonah that reflected the dilemmas of being human. Thus, they did not begin theological reflection, reflection about God, from philosophy or theory, they were informed instead of their experiences of being human and being a nation among the nations. The Church took over this reasoning when it wrote in the New Testament about the experience of the life and death of Jesus.
This approach to theology held theology to account. Theology could not be mere wishful thinking, it had to be grounded, it had to take into account human experience, especially the experience of failed religious systems that crumbled under the weight of human witness.
Religious notions come up against reality. The narrative of the human that we find in Scripture is a narrative of failure as Israel faced exile in Babylon after the fall of Jerusalem and the destruction of the temple.
Similarly, the New Testament has at its centre the tragedy of the crucifixion and death of Jesus and the apparent emptying of all hope and meaning. Christian theology does not emerge from a hopeful attitude or a triumphant religious sentiment. It emerges from a cry from the cross by one who we had hoped would be the Messiah, the anointed. This seditious cry: "My God, my God, why have you forsaking me?" marks the lowest point of human experience where darkness rules over all. If a kindly Father God was looking down from above ready to intervene for his Son he must have turned aside so as not to see.
Christian theology regards the life and death of Jesus as the objective basis for faith. As is apparent from the above, it took its cue from the Hebrew Scriptures. This beginning point is quite different from beginning with a notion of God derived from a perceived order in the universe or from philosophical speculation or, indeed, from a survey of world religions. The life and death of Jesus is the anchor in Christian theology that stops speech about God spinning off into self-serving speculation. While religion may be a story we spin to comfort ourselves, Christianity, has at its base an apparent failure, the scandalous death of the Messiah. It can never be understood and pursued as an antidote to our fear of death or meaninglessness. Rather, we are confronted by One who is "Other", One who speaks uncomfortable truths and who has no respect for our optimistic religious notions.
The Son in the Trinity denotes the historical experience of both Israel and the Church and is the starting point for all speech about God.
One would imagine that saying something about the Son would be easy. After all, Israel had a history and Jesus was an historical person. It would seem that we are on the safe ground of historical research and biography. This is far from the truth. Certainly, Israel had a history among the nations and Jesus was a man among men marked for death as are all men. But both Israel and the Church saw the character of God in the events that transpired.
Hence God is intimately connected with real events in the world, not as a being intervening in these events from the outside, as it were, but within the events themselves in the truth that they carried. Jesus comes to us in a form we least expect, he comes as a stranger and certainly not a creature of our own invention as we would expect if he were a made-up Messiah. The doctrine of the Trinity was the outcome of meditation on the relationship to these events in history, the truth that they carried and how that truth was present in all of time.
The narrative that the Church weaved in this respect became doctrine. For example, the event of the incarnation was not that a good man had been raised because of his righteousness to sit at the right hand of the Father but that the Father has sent the Eternal Word, the image of the Father Himself, to live among men and women.
This means that we must talk in the puzzling terms of the pre-existence of Jesus. This is a puzzle to anyone who sees only the man from Galilee. If we are to understand what the incarnation means we must deal with the idea of the two natures of Christ. Yes, he was a man who sweated in the midday sun and shared in the whole estate of humanity that included dying. However, the doctrine of the Trinity tells us more, that he was in the beginning with the Father as the prologue to John's gospel tells us. There was a truth about Jesus that went beyond his humanity so that he was the fulfilment of the character of God.