The Westminster Larger Catechism in modern English question number thirty-eight:
Q. 38. Why was it necessary that the Mediator should be God?
A. It was necessary that the Mediator should be God, first, so that He might sustain and keep the human nature from sinking under the infinite wrath of God and the power of death; second, so that He might give worth and efficacy to His sufferings, obedience, and intercession; and third, so that He might satisfy God’s justice, gain His favor, purchase a unique people, give His Spirit to them, conquer all their enemies, and bring them into everlasting salvation.