Q. 38. Why was it requisite that the mediator should be God?
A. It was requisite that the mediator should be God, that he might sustain and keep the human nature from sinking under the infinite wrath of God, and the power of death; give worth and efficacy to his sufferings, obedience, and intercession; and to satisfy God’s justice, procure his favor, purchase a peculiar people, give his Spirit to them, conquer all their enemies, and bring them to everlasting salvation.
It’s a detail list I suggest you find and read Vos’ commentary on your own. Here is a snippet.
How does the fact that Jesus Christ, the Mediator, is truly God guarantee the success of a plan of salvation? If Jesus Christ were only a human being even, a perfect human being, it would have been possible for him to fail in this work, by yielding to temptation and falling into sin. In that case the second Adam would have been it failure just as the first Adam was, by disobeying the will of God. But because Jesus Christ was not only human, but also truly divine, he was and is almighty. Therefore his success is a certainty, since he can neither fail in his work nor fall into sin.