In a previous post, we considered the way in which Geerhardus Vos’ doctrine of Christ impacted his redemptive-historical hermeneutic for reading the Old Testament. In the triune God’s eternal counsel of peace, the Son assumed his role as Mediator and Surety of the covenant of grace. Therefore, the Old Testament revelation that had him as its center and goal was never of him as the Logos in the abstract, but always as the Logos to be incarnate in time. For this reason the Old Testament revelation with its types had to point forward to Christ as the antitype. And not only did it point forward to the fullness of time (Gal. 4:4), but also heavenward. For the prophets, priests and kings were messengers and representatives of the great antitype, the eternal Son of God anointed as Mediator from eternity. “They derived their official authority from the person Himself whom they as office bearers proclaimed in a shadowy fashion” (Vos, Reformed Dogmatics, 3:90). This means that believers under the old covenant were not saved “otherwise than by the official activity of the Messiah” (90).
Building on this, we notice a further integration that Vos develops with Christology and Covenant: he grounds the stability and certainty of the covenant of grace in the hypostatic union. By this union we affirm that the divine person (the Logos) assumed a human nature. It was not the union of a divine person and a human person, but the union of the divine nature and a human nature in the divine person of the Logos. In possession of both a true humanity and true divinity, he was fully God and fully man, the God-man. This person, and no other, is the Mediator and Surety of the covenant of grace.
The question, then, is what impact does Christ being a divine person, the God-man, have on the covenant of grace? Or, how does the covenant of grace differ from the covenant of works by having Christ as its Mediator? While the church has always affirmed and defended the necessity of Christ being both truly God and truly man (see e.g., Heidelberg Catechism Lord’s Day 6), the implications are sometimes left unturned.
Vos will argue that the covenant of grace derives its certainty not in the abstract, but from the person of its Mediator.
Only because the divine person is the subject in Christ does His mediatorial work obtain the stability required by an eternal, immutable covenant of grace. We now know, however, that this human nature in itself is an abstraction that did not exist for a moment without personal subsistence in the Logos (48).
Note, first, the careful distinction Vos makes between person and nature. He is not saying that the attribute of immutability that belonged to the divine nature was communicated to the human nature. The divine nature remains divine and the human nature remains human. The unity of the two natures lies solely in the divine person of the Logos (see p. 42). Again, the Logos did not assume a human person but a human nature. On this basis, Vos can write, “[I]n Christ’s human nature there was not a mutable human person but the person of the Son of God. Will or intellect or emotion in the human nature could not have sinned unless the underlying person had fallen from a state of moral rectitude. There can naturally be no thought of the latter for the Mediator, considering the deity of His person” (58).
Second, note how Vos understands the covenant of grace as eternal and immutable to require a certain kind of mediatorial work, namely one that is stable. Where does this stability come from? Vos says it comes from the Mediator being a divine person; particularly, from the human nature subsisting in the Logos, the second person of the Trinity. “The human nature of the Mediator did not exist for an instant apart from the person of the Son” (62). In short, the immutable nature of the covenant of grace required the assumption of a human nature by none other than an immutable divine person.
So Vos goes on to say,
Thus the person of the Logos with its personality provides His human nature with the steadfastness and immutability by which the covenant of grace is distinguished from the first covenant, the covenant of works. The oneness and the deity of the person are of importance for the affirmation that Christ could not sin (48).
The impeccability of Christ that stabilizes the covenant of grace in its immutability is not owing to the deification of his humanity, but from the fact that his humanity subsists in a divine person. The covenant of works did not possess such stability because it did not have the God-man as its mediator. So while the covenant of works could be broken, the covenant of grace is indestructible.
The practical import of all this is that the immutable and guaranteed nature of the covenant of grace is given a concrete and real ground in the person of Christ himself. We do not affirm the certainty of God’s covenant in the abstract, but on the basis of who Christ is as its Mediator and Surety. The promise of God in the covenant of grace to be our God and for us to be his people is as unbreakable as the unity of the two natures in the divine person of the Logos. His two natures would first have to be ripped apart before the threads of God’s promise could be unravelled. The covenant of grace, in which we find the complete forgiveness of ours sins and eschatological fellowship with the triune God forever, is founded upon nothing less than divine omnipotence. So in Christ we can be absolutely sure that all of God’s promises are, in fact, Yes and Amen.
 For Vos’ critique of Lutheran Christology with respect to the communication of attributes see pp. 65–74, esp. 70ff.
 Vos asks, “Is this one subject, this one person in the Mediator, a divine or a human person?” He answers, “This person is divine, and not human or divine-human. In order to be immediately convinced of this, one may take the following into consideration. In the Logos, a divine person, who is immutable, is present from eternity. If now there can be but one person in the Mediator, and the divine person cannot be eradicated or changed, then it is self-evident that this one person is the divine person of the Logos. One can only maintain the immutability of God if one holds to the deity of the person in the Mediator. The choice lies between two persons or one divine person” (42).