WLC40

The WESTMINSTER LARGER Catechism

WLC 40. Why was it requisite that the Mediator should be God and man in one person?

A. It was requisite that the Mediator, who was to reconcile God and man, should Himself be both God and man, and this in one person, that the proper works of each nature might be accepted
of God for us,¹ and relied on by us, as the works of the whole person.²

Proofs
^{1Mt 1:21, 23; 3:17; Heb 9:14;²1 Pet 2:6
Comments
We saw in WLC 36 that the Lord Jesus Christ was fully God and fully man "in two entire distinct natures, and one [divine] person, for ever."
There was,—as we noted in our comments on WLC 36,—a heresy in the 5^th Century, known as Nestorianism which essentially taught that Christ was not one person but really two distinct persons, one human and one divine co-joined together. Nestorianism correctly distinguished between the deity and humanity of Christ, but insist that there was no real union of God and humanity in one divine person. The Son of God and the Son of Man are distinct persons. Though only the physical body of the Son of Man was manifested, the voice of both were heard.
The Council of Ephesus in A.D. 431 pronounced Nestorianism a Christological heresy and excommunicated Nestorius, the bishop of Constantinople, who allegedly taught it. The Ecumenical Council of Chalcedon in A.D. 451 reaffirmed the decisions of the Council of Ephesus.
WLC 40 essentially answers the question as to why it is important to insist that Christ was one person, and that Nestorianism was wrong.
Scripturally, the fact that Christ was one person can be seen in 5 ways:

Firstly, in the fact that the Scripture does not anywhere attribute any action, word, or attribute to Christ which suggests that He is other than a singular individual.

Secondly, Scripture does not use the plural pronoun for Christ, unlike the reference to the Triune God.

Thirdly, there are passages in which Christ is designated by a divine title while an attribute or action that properly belongs to His human nature is being describe. Thus we read of "the church of God, which He hath purchased with his own blood" (Acts 20:28). God is spirit, the only way God may be said to shed blood is to have the divine nature united with the human nature in one person.

Fourthly, there are passages in which the reverse is true, e.g.: "No man hath ascended up to heaven, but he that came down from heaven, even the Son of man which is in heaven" (Jn 3:13). The title ‘Son of man’ seems to highlight the humanity of Christ, yet it is with this term that He is said to have descended from heaven.

Fifthly, there are passages in which both divine and human attributes and actions are predicated to the same person, e.g.: "In whom we have redemption through his blood, even the forgiveness of sins: Who is the image of the invisible God, the firstborn of every creature: For by him were all things created…" (Col 1:14-16a); "For in him dwelleth all the fulness of the Godhead bodily" (Col 2:9).

The Answer to WLC 40 is no doubt founded on these scriptural bases, but is presented theologically on two points:
Firstly, we are told that our Mediator must himself be God and Man and in one person, so that "the proper works of each nature might be accepted of God for us."

Consider for example that Christ must needs suffer, but as God, He could not experience suffering except through a human nature; and His human nature could not endure the degree of suffering required except it be sustained by a divine nature. Therefore Christ must be God and Man at the same time, and he must be one person because he must suffer and yet must suffer infinitely. Therefore the writer of Hebrews, speaks of "the blood of Christ" offered "through the eternal Spirit [i.e. Christ’s divine nature]" (Heb 9:14). The blood of Christ would be of little value in God’s sight except it was the blood of man who is also God.

Secondly, we are taught that Christ must be God and Man, so that He might be "relied on by us, as the works of the whole person." Christ is our representative, He must be man but how can He mediate for us if He is not also God. Therefore in order that we might rely on Him, He must be one person who is both God and man. W}