Why do certain liberal scholars say that there are two Isaiahs?
The book of Isaiah is divided into two sections: Chapters 1–39 emphasize the enemy Assyria, and chapters 40–66 warn about the coming Babylonian captivity. Those who deny the authorship of Isaiah say the first Isaiah wrote the first thirty-nine chapters, and a second Isaiah wrote chapters 46–66. They say the predictions of chapters 40–66 couldn’t be known by the first human writer, so the liberals say the second Isaiah wrote after the Babylonian captivity occurred. Because liberals deny the supernatural, they have to explain away the predictions of the future.
What proof is there that there was only one Isaiah?
Throughout the book there is one primary title given to God: “the Holy One of Israel.” It occurs in both the first and second sections and reflects the unity of the book. The New Testament writers quoted both sections and credited both to Isaiah. Jesus Himself quoted both sections of the book and attributed them to Isaiah. If Jesus had made a mistake, then he could not be equal with God, or be God, because God is onmiscient (all-knowing), incapable of making a mistake.
Further, when Isaiah describes vegetation and geography, he is describing Palestine in both sections, not Babylon, where the liberals say the second part of Isaiah was written. Finally, when the Dead Sea Scrolls were discovered, there was no break in the book of Isaiah between the first and second half; if anything, the break occurred in the middle of a line in the middle of the page. There wasn’t even a paragraph break. Hence, those who study Isaiah carefully treat it as one book by one author.
Elmer Towns, Bible Answers for Almost All Your Questions (Nashville: Thomas Nelson, 2003).