No human sacrifice can pay the infinite debt we owe to the holy God. The sacrifice for our sins must be made by someone whose death can actually pay the debt. Then, there is the fact that no human can apply their sacrifice or obedience to another so as to pronounce forgiveness. Only God can apply the fruits of Jesus’ redemptive work to others under the terms of a covenant of grace, which he alone can establish on his oath.
Christians often speak of important doctrines in the abstract. People speculate about election and predestination, the purpose and extent of the atonement, and so on, without making any connection between these doctrines and the person and work of Jesus Christ. But the Bible does not allow us to do this. If we follow the biblical pattern and language, we cannot even mention the subject of election, without at the same time mentioning that we are chosen from before the foundation of the world in the person of Jesus Christ, who was himself chosen by the Father to be the redeemer of the world (John 17:1 ff.). The seed of the woman mentioned in the first gospel promise (cf. Genesis 3:15) is Jesus of Nazareth, who has redeemed us through his saving work. And so on. This is why the eternal son of God became incarnate–to save his people from their sins. And this brings us back to the covenant of grace, and its mediator, Jesus Christ.
It is important to carefully consider the fact that the covenant of grace has a personal mediator–Jesus Christ. He is revealed to us in the types and shadows of the Old Testament, specifically through Moses’ office as mediator of that covenant God makes with Israel at Mount Sinai, as well as through the kingship of David and his rule over Israel. He is even revealed in the sacrifices for sin offered to God by the priests of Israel. All of these Old Testament events foreshadow the coming of God in human flesh. This is why whenever we speak about election and covenant, we must focus upon the word made flesh. Election, covenant, and incarnation are inseparable, and can only be explained and understood in the light of the coming of Jesus Christ, who is “God with us.” This same Jesus is also the mediator of the covenant of grace which progressively unfolds through the pages of the Old Testament, and is fulfilled in the New.
As the promise of redemption begins to unfold, it is clear that God’s promises will be fulfilled in a single person, that one who is the seed of the woman, who will be Israel’s ultimate prophet, priest, and king, and who will also serve as the one mediator between God and man (cf. 1 Timothy 2:5). This one person and mediator of the covenant will be truly human, and yet truly God. He will have two distinct natures, yet he remains one person.
The two natures of Jesus Christ points us to the lengths to which a gracious God will go to save us from our sins.