The questioning began in the nineteenth century. At first interpreters believed that Isaiah of Jerusalem of the eighth century only wrote chapters 1-39 (or less), and that a second unknown prophet wrote the second part (chapters 40-66) in Babylon sometime before or at the time of the return from exile in the sixth century BC. Later some interpreters limited the Deutero-Isaiah to chapters 40-55and attribute chapters 56-66 to a third author (Trito-Isaiah) in Palestine after the return from exile, maybe in the fifth century BC. What does the evidence indicate? Upon examination, both external and internal evidence seem to point to a single author, who wrote as the Holy Spirit inspired him and with his style and personality reflected in his writing. .
External Evidences.
1. Uniform ancient Hebrew tradition supports the authorship and unity of Isaiah. Isaiah was seen as one book by one author by ancient Hebrew scholars. All latter prophetic books begin with the author's name, and Isaiah has one name and one beginning in all known manuscripts and versions. This includes the Isaiah scroll discovered at Qumran and dated in the mid-second century BC, the testimony of the Septuagint, and other ancient versions. Josephus, a first-century Hebrew historian, also saw it as one book. .
2. Uniform Christian tradition indicates one book by one author. From the writers of the early church up until the eighteenth century there was complete agreement without question on the unity of Isaiah, approximately 2500 years. When humanism, naturalism and skepticism became the norm for humanity in the last half of the eighteenth century, questions arose about Isaiah's unity, the authorship of the Pentateuch, the date of Daniel, the historicity of Jonah, and other questions about the integrity and authority of God's Word. .
3. The testimony of New Testament writers supports unity. They quote from all parts of the book of Isaiah and attribute their quotes to Isaiah as the single author.