Prima Facie evidence
The primary evidence is that the text claims to have been written by Isaiah the son of Amoz, and that his career spanned portions of the reigns of Kings Uzziah, Jotham, Ahaz, and Hezekiah (see Isaiah 1:1), putting us in the 8th century BC.
The secondary evidence is that Jesus and His apostles quoted the words of Isaiah and exhibit the belief that Isaiah wrote them.
The tertiary evidence is that 100% of the extant manuscript evidence supports Isaiah as a unified work, rather than a set of separate texts that were later compiled.
Against this unanimous set of evidence the question might be restated, what evidence is there that the text wasn't written by Isaiah ben Amoz around the 8th century BC?
Dead Sea Scrolls
I suggest the Dead Sea Scrolls work against the multiple-Isaiahs/late Isaiah theories:
- They show that although the Jewish scribes weren't 100% perfect, they were exceptionally meticulous in accurately copying their sacred texts
- The Great Isaiah scroll is not claimed by anyone to be the original manuscript--it's a copy of a copy of copy etc...so the fact that the earliest surviving manuscript is not from the 8th century BC says nothing about when the original was written. Very nearly all Greco-Roman writings, some of which are considered very historically reliable, come to us through copies made centuries after the originals were composed
- The Great Isaiah scroll has essentially the entire book of Isaiah - it's one work attributed to one author, not three works stitched together
Those who assume the impossibility of supernatural prophecy will naturally (no pun intended)(okay, maybe it was) assert that a statement that refers to King Cyrus of Persia by name (6th century BC) could not have been written until after the fact. This is a circular argument--it treats what it wants to prove as an axiom and then claims it is therefore true. I offer a more extensive critique of this argument on my channel here.
We could assume in advance--before even looking at the evidence--that Isaiah could not be a prophetic document, but this is neither scientific nor is it an argument--it is a philosophical assertion.
The Book of Isaiah can be divided into (at least) 3 sections--one of the most popular descriptions is:
- The Assyria section (chapters 1-35)
- The history section (chapters 36-39)
- The Babylon section (chapters 40-66)
Other subdivisions exist, but this is a pretty good generic outline.
The argument goes that the subject matter is so different in the "Babylon" section versus the "Assyria" section that this must be the work of different people. On closer inspection, however, there are multiple possibilities here--let's focus on the two simplest:
- These are different authors who each always wrote about the same things.
- This is one author who has written on several topics, and has organized his book by grouping the material by topic. (a writer & poet of this caliber was clearly well-read, and probably had thoughts on multiple topics)
The Occam's razor approach would be option 2, which does not multiply entities beyond necessity. For those who hold to option 1, I am quite curious whether they would apply the same source-critical reasoning to all books that are organized by subject matter...must they all be the products of multiple authors as well?
The preponderance of the evidence supports the view that Isaiah was a highly-literate author living in the 8th-century BC, and that the full book of Isaiah is his work.
This view has been unpopular in academic circles for the last few centuries, because it is at odds with naturalism. Those who prefer to accept naturalism without question must reject traditional authorship of Isaiah regardless of the evidence.