Westminster Larger Catechism in modern English question number forty:
Q. 40. Why was it necessary that the Mediator should be God and a human in one person?
A. It was necessary that the Mediator, who was to reconcile God and mankind, should Himself be both God and a human—and this in one person—so that the proper works of each nature might be accepted by God for us and relied on by us as the works of the whole person.