Heidelberg Catechism Q. 18 asks:
18. But who now is that Mediator, who in one person is true God and also a true and righteous man?
Our Lord Jesus Christ, who is freely given unto us for complete redemption and righteousness.
The Mediator cannot be two persons. He must be one person with two natures. In 1530, in the Augsburg Confession, the Protestants confessed:
Also they teach that the Word, that is, the Son of God, did assume the human nature in the womb of the blessed Virgin Mary, so that there are two natures, the divine and the human, inseparably enjoined in one Person, one Christ, true God and true man, who was born of the Virgin Mary, truly suffered, was crucified, dead, and buried, that He might reconcile the Father unto us, and be a sacrifice, not only for original guilt, but also for all actual sins of men (Article 3).
In the Belgic Confession (1561), the Reformed Churches confess:
We believe that by this conception the person of the Son is inseparably united and connected with the human nature; so that there are not two Sons of God, nor two persons, but two natures united in one single person; yet each nature retains its own distinctive properties. As, then, the divine nature has always remained uncreated, without beginning of days or end of life, filling heaven and earth, so also has the human nature not lost its properties but remained a creature, having beginning of days, being a finite nature, and retaining all the properties of a real body. And though he has by his resurrection given immortality to the same, nevertheless he has not changed the reality of his human nature; forasmuch as our salvation and resurrection also depend on the reality of his body.
In the years between the Augsburg and the Belgic, a significant fissure had developed between the way the Reformed and the Lutherans explained the “one person, two natures” formula. The question is explained in the Epitome of the Formula of Concord (1580):
The chief question, however, has been whether, because of the personal union, the divine and human natures, as also their properties, have realiter, that is, in deed and truth, a communion with one another in the person of Christ, and how far this communion extends.
The Lutherans characterized the Reformed with the epithet, “sacramentarian” referring to the etymology of the Latin noun sacramentum as a military oath, which Zwingli had emphasized in his dispute with Luther in the 1520s-31. They accused the “sacramentarians” of asserting that the personal union of the divine and human natures is such that “neither has “realiter “in deed and truth” anything “in common with “that which is peculiar to either nature.” Thus, they complained, the Reformed view “makes nothing more than the names common…” .
The Reformed and the Lutherans defined “personal union” differently. In effect, if the Reformed disagreed with the Lutherans, they denied a real union. The Reformed version of the personal union was mere nominalism. For the Lutherans, what can be said of the divinity can also be said of the humanity. What can be said of the person can be said of either nature. The Reformed, in contrast, wanted to preserve both the personal union of the two natures and the distinctions between the two natures.
Thus, the Heidelberg Catechism, Q. 48 says:
Since his human nature is not present wherever His Godhead is, are not then these two natures in Christ separated from one another?
Not at all; for since the Godhead is incomprehensible and everywhere present, it must follow that the same is not limited with the human nature He assumed, and yet remains personally united to it.
The Reformed were concerned that the Lutheran approach to the communication of properties communicatio idiomatum placed in jeopardy the Chalcedonian formula, “consubstantial with the Father according to the Godhead, and consubstantial with us according to the Manhood; in all things like unto us, without sin.” That consubstantiality with our humanity meant that just as we are locally present, so too Christ is locally present in his humanity at the right of the Father. Ubiquity is a property of the deity, not the humanity. Locality is a property of the humanity, not the deity.
The Definition of Chalcedon summarized,
inconfusedly, unchangeably, indivisibly, inseparably; the distinction of natures being by no means taken away by the union, but rather the property of each nature being preserved, and concurring in one Person and one Subsistence, not parted or divided into two persons, but one and the same Son….
The nature of the personal union is a mystery. This is why Chalcedon employed a series of significant adverbs. According to Chalcedon, each nature remains what it was. The personal union does not change the nature substantially, yet there is one subsistence, not two, one person, not two.
According to the catechism, the formula one person, two natures (as defined by Chalcedon) is no mere theory. It is essential to our salvation. It was humanity that sinned but it was only God who could save. When the catechism says “Mediator” it means “federal representative.” The same human nature that sinned had to perform righteousness but only God the Son could, as the catechism says, sustain the burden of God’s wrath and redeem us from it. We do not have two Mediators, two federal heads, two representatives but one.
Our assurance rests in the reality of God the Son incarnate and in the obedience he performed for us which has been imputed to us and, as the Belgic reminds us, our hope the resurrection rests in the consubstantiality of our humanity with his.