Christ is Mediator, not because He is the Word, for as the Word He is supremely blessed and supremely immortal, and therefore far from miserable mortals; but He is Mediator as He is man, for by His humanity He shows us that, … the blessed and beatific God, having Himself become a partaker of our humanity, has afforded us ready access to the participation of His divinity. For in delivering us from our mortality and misery, He does not lead us to the immortal and blessed angels, so that we should become immortal and blessed by participating in their nature, but He leads us straight to that Trinity, by participating in which the angels themselves are blessed. Therefore, when He chose to be in the form of a servant, and lower than the angels, that He might be our Mediator, He remained higher than the angels, in the form of God,—Himself at once the way of life on earth and life itself in heaven.
— City of God Bk. IX Ch. 15
As the things beneath, which are mortal and impure, cannot hold intercourse with the immortal purity which is above, a mediator is indeed needed to remove this difficulty; … We need a Mediator who, being united to us here below by the mortality of His body, should at the same time be able to afford us truly divine help in cleansing and liberating us by means of the immortal righteousness of His spirit, whereby He remained heavenly even while here upon earth. Far be it from the incontaminable God to fear pollution from the man He assumed, or from the men among whom He lived in the form of a man. For, though His incarnation showed us nothing else, these two wholesome facts were enough, that true divinity cannot be polluted by flesh, and that demons are not to be considered better than ourselves because they have not flesh.
— City of God. Bk. IX. Ch. 17
True Mediator, in so far as, by assuming the form of a servant, He became the Mediator between God and men, the man Christ Jesus, though in the form of God He received sacrifice together with the Father, with whom He is one God, yet in the form of a servant He chose rather to be than to receive a sacrifice, that not even by this instance any one might have occasion to suppose that sacrifice should be rendered to any creature. Thus He is both the Priest who offers and the Sacrifice offered. And He designed that there should be a daily sign of this in the sacrifice of the Church, which, being His body, learns to offer herself through Him.
— City of God. Bk. X. Ch. 20