All this means that any christological approach that starts from the man Jesus, from the historical Jesus, and tries to pass over to God, and so to link human nature to God, is utterly impossible. In fact it is essentially a wrong act: for it runs directly counter to God's act of grace which has joined God to humanity in Christ. All Attempts to understand Jesus Christ by starting off with the historical Jesus utterly fail; they are unable to pass over from man to God and moreover to pass man to God in such a way as not to leave man behind all together, and in so doing they deny the humanity of Jesus. Thus though Ebionite christologies all seek to go from the historical Jesus to God, they can make that movement only by denying the humanity of Jesus, that is by cutting off their starting point, and so they reveal themselves as illusion, and the possibility of going from man to God is revealed as likewise illusory.
No, it is quite clear that unless we are to falsify the facts from the very start, we must face with utter and candid honesty the New Testament presentation of Christ to us, not as a purely historical figure, nor as a purely transcendental theophany, but as God and man. Only if we start from that duality in which God himself has already joined God and man, can we think God and humanity together, can we pass from man to God and from God to man, and all the time be strictly scientific in allowing ourselves to be determined by the nature of the object. [Thomas F. Torrance, Incarnation, edited by Robert Walker, 10]
So Torrance's premise is that what has happened in the incarnation is totally unique, and has only been made accessible through the purview of faith. Not blind faith, but faith that takes it shape as it is given to us through the revelation of God himself in and through the vicarious humanity of Christ for us; the humanity that grounds all of humanity as the image of God (Colossians 1:15)--so faith is the eyes to see and the ears to hear with that Jesus so often challenges his audiences to employ.
What does this tell us about historical Jesus studies that seek to tell us the truth about Jesus, but then fail to do so through the mode of faith? It tells me that these approaches are trying to find a public square, a common ground through which to substantiate and situate Jesus in such a way that he has respectability amongst the world. Once this respectability has been established, and Jesus rightly reconstructed, then we can attend to the issues of faith (so there is an implicit competition, then, in this scenario, between the Jesus of Faith and History). To be clear, none of this is to reject the usage of historical tools, but it is to call attention to the need that these tools have; the need is to provide for them a prior Christian dogmatic order that will allow those tools to not chisel a Jesus into something he is not (e.g. first a man, then God added on). He is God first, who becomes man; and this is such a unique event that it in itself can only be its own analogy.
There is more to say about the TF Torrance quote above; especially about the affirmation of creation and humanity that is provided for it through the incarnation of Christ. Maybe another time, or in the comments.