"For that which he has not assumed he has not healed; but that which is united to the Godhead he has saved" (Letter 101)In other words, if Jesus did not become completely human like us then all that he achieved is ineffective and so a waste of time and energy. Humanity remains in the condition that it has been in since the Fall and we need an alternative saviour to either "top up" what Jesus didn't (or wasn't able to) achieve, or have another go at it all. Clearly this will not do.
The second problem is the relationship between sin, the "human condition", and the Godhead. Theologically we believe that the Logos, the second person of the Trinity, unites with humanity in order to redeem it in the Incarnation. But the close proximity then of God and sin becomes a theological conundrum that needs to be dealt with. How can God and sin co-exist? Is it possible? Of course we would say "No, it can't".
So how do we answer the questio; "What kind of human was Jesus?" Was he truly like us in every way, yet without sin? Or was he just something like superman. They thought he was just Clark Kent, but he turned out to be something from another world!
I would like to suggest that answering this Christological question will be easier if we learn how to answer another anthropological question better. We need to ask the question "What do we mean by human?" before we ask "What kind of human was Jesus?" This is a big question to address and an aspect of theology that I'm very interested in, but for now I'm just going to put it out there and skip over answering it. I will return to this question another day.
The next step is to tie together as closely as is possible the Incarnation and the Atonement. We should not separate who Jesus is from what Jesus does. In simple terms, we tend to want to keep Christmas and Easter separate. We shouldn't. In fact, we can't. One means nothing without the other, and indeed everything that lies in between the two is intrinsic to the Christian faith. The result of this is that the conception, birth, life and ministry of Jesus are just as important as that which is achieved on the cross. The same is true of the resurrection and ascension of Christ. As important as the cross is, it means nothing if we don't know who was on it and who defeated death by it.
The theologian I'm researching at the moment, Gregory of Nyssa, summaries this thought much better than I ever could, so I'd like to share his thoughts on this important matter.
We say that the Only-begotten God, having by His own agency brought all things into being, by Himself has full power over all things, while the nature of man is also one of the things that were made by Him: and that when this had fallen away to evil, and come to be in the destruction of death, He by His own agency drew it up once more to immortal life, by means of the Man in whom He tabernacled, taking to Himself humanity in completeness, and that he mingled His life-giving power with our mortal and perishable nature, and changed, by the combination with Himself, our deadness to living grace and power. And this we declare to be the mystery of the Lord according to the flesh, that He Who is immutable came to be in that which is mutable, to the end that altering it for the better, and changing it from the worse, He might abolish the evil which is mingled with out mutable condition, destroying the evil in Himself. For "our God is a consuming fire," by whom all the material of the wickedness is done away. (Gregory of Nyssa. "Against Eunomius." In Nicene and Post-Nicene Fathers, edited by Philip Schaff, vol 5, (New York: Christian Literature Publishing Co, 1892, 33-248) Book 5, 179.)It is clear here that there is an inseparable connection between Incarnation and Atonement; who Jesus is and what Jesus does. As a result, by virtue of the belief that it is God "co-mingling" with humanity within Jesus himself, Jesus sanctifies humanity - both his own human flesh and ours. Where there was the stain of sin, Jesus cleanses. Where there was guilt, Jesus brings forgiveness. Where there was sickness, he brings healing. Where there was slavery to sin, he brings freedom, and so on. I could continue, but the point is clear. He does this because he is truly and properly God, but it is effective because he is also truly and properly human.