Westminster Larger Catechism Question 40

Q. Why was it requisite that the Mediator should be God and man in one person? A. It was requisite that the Mediator, who was to reconcile God and man, should himself be both God and man, and this in one person, that the proper works of each nature might be accepted of God for us, and relied on by us, as the works of the whole person.

Calvin made the same point discussing Romans 1:3.

Who was made, etc. — Two things must be found in Christ, in order that we may obtain salvation in him, even divinity and humanity. His divinity possesses power, righteousness, life, which by his humanity are conveyed to us. Hence the Apostle has expressly mentioned both in the Summary he gives of the gospel, that Christ was manifested in the flesh — and that in it he declared himself to be the Son of God. [source]