The force of the proof lies in the agreement of the apostolic preaching with the good tidings of the heralds foreseen by the Prophet. What did the Apostles preach? Peace with God.
They preached such peace that from it flowed countless spiritual blessings, both for time and still more for eternity. Saint Paul says of this: God gave us the ministry of reconciliation. He placed upon us the word of reconciliation.
"Therefore, on behalf of Christ we beseech you, as though God were beseeching through us. We beseech you on behalf of Christ: be reconciled to God" (cf. 2 Cor. 5:20).
Here, after citing the prophetic word about the heralds of peace, he speaks as though saying: recognize in us those heralds foreseen by the Prophet. Do not say that, since there was no preaching and no preachers, I could not hear and believe.
"Do you see," says Saint Chrysostom, "how by the very manner of the preaching the Apostle proves that they are the preachers, the ones foreseen and sent? The Apostles, going throughout the inhabited world, proclaimed nothing other than ineffable blessings and God's peace with men.
"Therefore you unbelievers do not disbelieve us, Paul says, but Isaiah. Many years beforehand he proclaimed that we would be sent, that we would preach, and he said the very thing that we say.
"If salvation depends on calling, calling on faith, faith on hearing, and hearing on preaching, and if the Apostles were sent and preached, then the Prophet even went together with them.
"He pointed them out, proclaiming and saying: these are the very ones about whom it was announced from above through me long beforehand. I praised their feet because of the manner of their preaching.
"It is therefore evident that unbelief is the Jews' own fault, and that on God's part everything has been done."
In the passage from which the prophetic word is taken, the prophet Isaiah inspires the Jews with hope of deliverance from captivity. The ten tribes were already in captivity among the Assyrians.
The captivity of the rest by Nebuchadnezzar was impending, and the Prophet saw it as though already accomplished. So that they might not lose heart, the Prophet in the name of the Lord promises them deliverance: "Arise, O Zion, put on your glory, O Jerusalem" (cf. Is. 52:1).
Then the prophetic spirit carries the Prophet into the distant future, to the time of the coming of the Promised One. It compels him to speak in God's person: "Because of you My name is blasphemed among the nations" (cf. Is. 52:5).
But the time will come when "My people shall know My name" (cf. Is. 52:6). How shall they know it? I Myself, who speak this, will be among them.
Heralds will be sent, carrying the joyful message concerning Me. This is expressed in the words: "How beautiful are the feet of those who preach peace, who preach good things" (cf. Is. 52:7).
Then God will reign, and all the ends of the earth shall see the salvation that is from God (cf. Is. 52:10). Here the indication of Christ Jesus, the Lord and Savior, is so clear that the Jews easily recognized a prophetic indication of the time of His coming.
Ambrosiaster applies this passage to the time of the apostolic preaching: "By 'feet' he signifies the going of the Apostles, who traverse the whole world and preach that the Kingdom of God has come.
"Their going sanctified men, showing them the way by which one walks in peace toward God. John the Baptist first came to prepare this way. This is the peace toward which those who believe in Christ hasten.
"The Kingdom of God is peace. In it, every discord having been cut off, all bend the knee to the one God. So it is here. The Jerusalem above, which is our mother, by its very name is interpreted as the vision of peace, or enjoyment of peace."