Who hath believed our report? and to whom is the arm of the Lord revealed?
These two questions remain as timely today as they were when asked 750 years before Christ's birth. As to the first question:
The report is Isaiah's testimony of the coming Messiah.
It is plural, although delivered by a lone prophet, because God Himself authorized the message to be delivered. Therefore it is not "my" but rather "our" report.
The question concerns the audience's "belief" in the report, because it contradicts the ideas held by them. It will tell them something remarkably different from what they though to be true.
As to the second question:
"The arm of the Lord" is a symbol of His strength or might.
To have the strength of the Lord revealed to someone is to have them come into knowledge of Him and His ways.
His ways are not what men presume they are. They are directed to much higher, much holier ends. The strength of the Lord as it will unfold in the chapter which follows is based upon the suffering He undertook for us.
The chapter that follows this opening verse is framed in the past tense. This is called the "prophetic perfect" tense. To the prophet, the events have been seen. To him, they are in the past. Therefore, future events are framed as if they already occurred. Prophets to whom things are shown will often frame their message in the past tense, even though they speak of things in the future. You find it throughout prophecy.