God and man in one.

John Calvin has written:

“Another principal part of our reconciliation with God was, that man, who had lost himself by his disobedience, should, by way of remedy, oppose to it obedience, satisfy the justice of God, and pay the penalty of sin. Therefore, our Lord came forth very man, adopted the person of Adam, and assumed his name, that he might in his stead obey the Father; that he might present our flesh as the price of satisfaction to the just judgment of God, and in the same flesh pay the penalty which we had incurred. Finally, since as God only he could not suffer, and as man only could not overcome death, he united the human nature with the divine, that he might subject the weakness of the one to death as an expiation of sin, and by the power of the other, maintaining a struggle with death, might gain us the victory. Those, therefore, who rob Christ of divinity or humanity either detract from his majesty and glory, or obscure his goodness. On the other hand, they are no less injurious to men, undermining and subverting their faith, which, unless it rest on this foundation, cannot stand. Moreover, the expected Redeemer was that son of Abraham and David whom God had promised in the Law and in the Prophets. Here believers have another advantage. Tracing up his origin in regular series to David and Abraham, they more distinctly recognize him as the Messiah celebrated by so many oracles. But special attention must be paid to what I lately explained, namely, that a common nature is the pledge of our union with the Son of God; that, clothed with our flesh, he warred to death with sin that he might be our triumphant conqueror; that the flesh which he received of us he offered in sacrifice, in order that by making expiation he might wipe away our guilt, and appease the just anger of his Father.”

(Calvin, John. Institutes of the Christian Religion, Grand Rapids, MI: Wm. B. Eerdmans Publishing, 1998, p. 402.)