The people of Israel are apostate, rebellious, and corrupt; only a few remain faithful—The people’s sacrifices and feasts are rejected—They are called upon to repent and work righteousness—Zion will be redeemed in the day of restoration.
Either he started writing all of this after the reigns of the kings, Isaiah didn't write it, or he prefixed it later. See also Isaiah 2:1, which appears in the Book of Mormon. In Isaiah 6:1 and Isaiah 8:1 he speaks of himself in the first person.
He gives his testimony, after the prophecy, that it is of the Lord.