From the Explanations of the Psalms by Saint Ambrose, bishop
The man Christ Jesus, the one mediator between God and men
Brother cannot redeem brother, but a man will redeem man. No one can give to God the ransom for himself nor the price of his soul’s redemption. Christ is saying: What have I to fear in the day of evil? What can do me harm if I do not need a redeemer but am myself the redeemer of all mankind? Shall I free others, yet tremble for myself? See, I shall make all things new, so as to surpass even the love and devotion of brothers. Where a brother, born of the womb, cannot redeem, suffering as he does from the infirmity of a common nature, yet a man will redeem, that man of whom it is written: The Lord will send them a man who will save them; the man who said of himself: You seek to kill me, a man who has spoken the truth to you.
He is a man, yet who will recognize him? Why will no one recognize him? Because, as there is one God, so there is one mediator between God and men, the man Christ Jesus. He alone will redeem man, showing love greater even than that of brothers. He poured out his blood for strangers, as no one is able to do for a brother. He did not spare his own body in redeeming us from sin, but gave himself as the redemption of all, and Paul the apostle is a true witness to him: I speak the truth and do not lie.
But why will this man be the only redeemer? Because no one can equal him in the love he showed in laying down his life for his own poor servants. Nor can anyone equal him in sinlessness, for all men are ruled by sin, and all are victims of the fall of the first Adam. He alone is chosen to redeem, for he alone cannot be subject to that age-old sin. So let us understand by “the man” the one who took upon himself the condition of man in order to crucify in his own flesh the sin of all, and to cancel by his own blood the debt owed by all: the Lord Jesus.
You may ask: How can we say that brother cannot redeem when the man we are discussing has said: I shall declare your name to my brothers? But it was not as our brother but as the man Christ Jesus, in whom God dwelt, that he forgave our sins. For it is written that God was in Christ, reconciling the world to himself. God was in the man Christ Jesus, of whom alone it was said: The Word became flesh and dwelt among us. It was not, therefore, as a brother but as the Lord that he dwelt among us in the flesh.