Christians often speak of important doctrines in the abstract. People speculate about election and predestination, the 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 (cf. Genesis 3:15) is Jesus of Nazareth, who has redeemed us through his saving work. 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–who is revealed to us in the types and shadows of the Old Testament 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, and even through 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. Given the fact of human sin, there is no possible way for men and women to be delivered from that sin apart from the incarnation of our Lord. Since it is the eternal Son of God, the Word, who became flesh, we must deal with the two natures of Christ and the way in which they are related in one person–Jesus. Since it was human nature which sinned, it was absolutely necessary for God to come to earth in the person of Jesus Christ so that someone truly human might do what is necessary to save us. The debt we owe to God must be paid by one of Adam’s race. And then the punishment involved in the curse (death) requires the ability to suffer in both body and soul. For such a penalty to be paid, the eternal Son of God must become flesh.
And yet, at the same time, if we are to be redeemed from our sin, the redeemer must himself be without the guilt of sin so that he could in fact make a sacrifice for sins on behalf of those whom he came to save which satisfies the holy justice of God. A sinful man with a sinful nature is not fit to save other sinners. This is why Jesus was miraculously conceived in the womb of the virgin by the power of the Holy Spirit, so that he was free from the guilt of Adam’s sin. This is also why Jesus subjected himself to the law of God (Galatians 4:4-6), to earn a justifying righteousness for his people through his own perfect obedience. This is why Paul calls Jesus the second Adam (cf. Romans 5:12-19). Jesus does not give into temptation like Adam did. He obeys all of God’s commands, unlike Adam. Jesus earns the crown of life which Adam failed to do. Jesus did all of those things, perfectly, and he did them as someone who was truly human. This is why Jesus is a better mediator than Moses, with a covenant much better than the one God made with Israel at Sinai.
Yet 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. This is why Jesus must also be fully God.
Since Jesus is truly man, he truly redeems human nature–which he took with him when he ascended into heaven after his resurrection. Not only do we have hope that the Son of God will redeem our souls, the two natures united in one person also means that God will redeem our bodies as well. Jesus not only has paid for our sins, and provides for us a perfect righteousness, presently he is in heaven in a body of glorified flesh–a body in which he will return to raise the dead, judge the world and make all things new. Because our Lord has redeemed human flesh, we are certain that our flesh will likewise be transformed so that we might live with him forever and ever.
This is why Jesus Christ must be true God and true man, yet one person who is the mediator of the covenant of grace. Jesus came to save us from our sins and to earn a perfect righteousness by perfectly obeying the law of God. But the Word also became flesh to redeem fallen human nature–and this required him to become like us in every way, yet without sin. When we speak of being saved–we mean much more than the salvation of our souls. We mean the redemption of our whole person–body and soul. This is why he is true God and true man, and the mediator of a better covenant.