Who hath believed? Isaiah felt that he spoke, mainly, to unbelieving ears (see above, Isaiah 28:9-15; Isaiah 29:10-15; Isaiah 30:9-11; Isaiah 42:23, etc.). The unbelief was likely to be intensified when so marvellous a prophecy was delivered as that which he was now commissioned to put forth.
Still, of course, there is rhetorical exaggeration in the question, which seems to imply that no one would believe. Our report; literally, that which has been heard by us. But the word is used technically for a prophetic revelation (see Isaiah 28:9, Isaiah 28:19; Jeremiah 49:14).
Here it would seem to refer especially to the Messianic prophecies delivered by Isaiah. To whom is the arm of the Lord revealed? The "arm of the Lord," which has been "made bare in the eyes of all the nations" (Isaiah 52:10), yet requires the eye of faith to see it.
Many Jews would not see the working of God's providence in the victories of Cyrus, or in the decision to which he came to restore the Jews to their own country. Unbelief can always assign the most plainly providential arrangements to happy accident.