The Enchiridion by St. Augustine
Chapter 108.--A Mediator Was Necessary to Reconcile Us to God; And Unless This Mediator Had Been God, He Could Not Have Been Our Redeemer.
For we could not be redeemed, even through the one Mediator between God and men, the man Christ Jesus, if He were not also God. Now when Adam was created, he, being a righteous man, had no need of a mediator. But when sin had placed a wide gulf between God and the human race, it was expedient that a Mediator, who alone of the human race was born, lived, and died without sin, should reconcile us to God, and procure even for our bodies a resurrection to eternal life, in order that the pride of man might be exposed and cured through the humility of God; that man might be shown how far he had departed from God, when God became incarnate to bring him back; that an example might be set to disobedient man in the life of obedience of the God-Man; that the fountain of grace might be opened by the Only-begotten taking upon Himself the form of a servant, a form which had no antecedent merit; that an earnest of that resurrection of the body which is promised to the redeemed might be given in the resurrection of the Redeemer; that the devil might be subdued by the same nature which it was his boast to have deceived, and yet man not glorified, lest pride should again spring up; and, in fine, with a view to all the advantages which the thoughtful can perceive and describe, or perceive without being able to describe, as flowing from the transcendent mystery of the person of the Mediator.