Ever since the 1700’s critical and liberal scholars have had a field day with the authorship of Isaiah. I say “ever since then” because before that time no one seriously questioned the authorship or unity of the book. In the 1700’s scholars decided that Isaiah was written by two people, and divided the book into two sections. They say the first section, 1-39, was written by Isaiah himself and the last section, 40-66, was written by a disciple of Isaiah often called “deteuro-Isaiah”. This began the de-coiling of the unity of Isaiah. The unity further de-coiled in the 1800’s with a new hypothesis. Scholars then said 3 people wrote Isaiah. 1-39 was by Isaiah, 40-55 by deteuro-Isaiah, and 56-66 by tripto-Isaiah, which means a “third” Isaiah. As well as making these separations in authorship, the dates they were giving Isaiah were moving earlier and earlier. They began around 700 BC, and now they’ve moved up to around 450-400 BC.
In our modern day, there has been a good progression in thought concerning these things. Rather than trying to show that the book of Isaiah is not unified, there have been many efforts to show that Isaiah is a unified work. Modern critical scholars have said that there is a unity within Isaiah, but there is a twist to its unity. Rather than saying that Isaiah wrote the whole thing himself, they say that the book was redacted by a group of Isaiah’s disciples, who collected his works and put them together in a collection. The number of authors continues to be added on as well. Conservative scholars have mostly remained the same throughout the years. They think Isaiah wrote the whole book himself (even the second and third sections), which brings a complete unity to the book. But they do think that Isaiah could have began his book (1-39) around 700 BC and finished it (40-66) around 500-400, because of the Assyrian nature of the first section and the Babylonian nature of the second. Now, I should say that 1-39, 40-55, and 56-66 do indeed sound different, but do we have to assume from this that someone other than Isaiah wrote these later sections? Not at all.
I think there is a deeper issue at stake as well. If someone dates the book of Isaiah post-exile rather than pre-exile, the nature of prophecy is impinged and the character of God is brought into question. Meaning that, the presupposed belief behind a later date (post-exile) of Isaiah is that God cannot predict the future, while the presupposed belief behind an early date (pre-exile) of Isaiah is that God can indeed predict the future. God claims to be able to do this all throughout Isaiah, especially in 40-55. Therefore if God cannot predict the future, when He claims to be able to do so, He cannot be trusted and is no different than the idols He contrasts Himself with in Isaiah 40-55. The questioning of God’s character that takes place when one gives Isaiah a later, post-exilic dating of the book is precisely why I think such a date is so dangerous. This is why I think an earlier, pre-exilic date of Isaiah is correct.