8 In the same prophet the inquirer would find clear proof that Christ is not merely one of the great men that have appeared in the world. For Jeremiah goes on to say: "Cursed be the man that trusts in man, and makes flesh his arm, and whose heart departs from the Lord: for he shall be like the heath in the desert, and shall not see when good comes; but shall inhabit the parched places of the wilderness, in a salt land not inhabited. Blessed is the man that trusts in the Lord, and whose hope the Lord is: for he shall be as a tree beside the water, that spreads out its roots by the river: he shall not fear when heat comes, but his leaf shall be green; he shall not be careful in the year of drought, neither shall cease from yielding fruit." On hearing this curse pronounced in the figurative language of prophecy on him that trusts in man, and the blessing in similar style on him that trusts in God, the inquirer might have doubts about our doctrine, in which we teach not only that Christ is God, so that our trust is not in man, but also that He is man because He took our nature.
So some err by denying Christ's humanity, while they allow His divinity. Others, again, assert His humanity, but deny His divinity, and so either become infidels or incur the guilt of trusting in man. The inquirer, then, might say that the prophet says only that Christ is God, without any reference to His human nature; whereas, in our apostolic doctrine, Christ is not only God in whom we may safely trust, but the Mediator between God and man— the man Jesus. The prophet explains this in the words in which he seems to check himself, and to supply the omission: "His heart," he says "is sorrowful throughout; and He is man, and who shall know Him?" He is man, in order that in the form of a servant He might heal the hard in heart, and that they might acknowledge as God Him who became man for their sakes, that their trust might be not in man, but in God-man.
He is man taking the form of a servant. And who shall know Him? For "He was in the form of God, and thought it not robbery to be equal to God." He is man, for "the Word was made flesh, and dwelt among us." And who shall know Him? For "in the beginning was the Word, and the Word was with God, and the Word was God." And truly His heart was sorrowful throughout. For even as regards His own disciples His heart was sorrowful, when He said, "Have I been so long time with you, and yet have ye not known me?"
"Have I been so long time with you" answers to the words "He is man," and "Have ye not known me?" to "Who shall know Him?" And the person is none other but He who says, "He that has seen me has seen the Father." So that our trust is not in man, to be under the curse of the prophet, but in God-man, that is, in the Son of God, the Saviour Jesus Christ, the Mediator between God and man. In the form of a servant the Father is greater than He; in the form of God He is equal with the Father.
Source: Reply to Faustus the Manichaean (New Advent)