5. What testifies to the unity of the book of Isaiah?
5 Though some have pointed to the book’s change of style from chapter 40 onward as indicating a different writer, or “Second Isaiah,” the change in subject matter should be sufficient to explain this.
... For example, the oneness of the book is indicated by the expression, “the Holy One of Israel,” which appears 12 times in chapters 1 to 39, and 13 times in chapters 40 to 66, a total of 25 times; whereas it appears only 6 times throughout the rest of the Hebrew Scriptures.