Good academic research is a function of making claims and supporting those claims with reasons and evidence. Up until the 18th century, preachers and believers lived by the claims about the book of Isaiah that had been made in the books of 2 Chronicles (32:20-23) and 2 Kings (20-8-11), which is that he was a prophet. The authors of those two books were Ezra and Jeremiah, respectively, who lived in the 5th and 6th centuries B.C., respectively. Isaiah lived in the 8th century B.C., well before them. Their claims to his prophet status were backed by the simple fact that his predictions came true before or during their lifetimes. In other words, their claims about him being a prophet were backed by reasons and evidence.
Isaiah prophesied a Babylonian takeover of Assyria (605 B.C.; Isaiah 14:24-25) and destruction of Solomon’s Temple (586 B.C.; Isaiah 63:18-19), which occurred before and during Jeremiah’s lifetime. He further prophesied the Mede and Persian rise to power (539 B.C.; Isaiah 47:11; 13:17-19; 20-9-10) and the Jewish exile and the return from the exile and the rebuilding of the Temple (Isaiah 27:12-13; 14:1-2; 45:13). These events occurred before Ezra’s lifetime. Jesus, John, Paul, and Luke all attributed a wide variety of Isaiah’s passages solely to the prophet Isaiah (e.g., John 12:36-43; Acts 8:28-38; Romans 10:16). In Against Apion 1:37-44 and Antiquities of the Jews, books 10 and 11, the 1st-century Jewish historian timed the closing of the Jewish canon of scripture to King Artaxerxes’ time (5th century B.C.) and attributed Isaiah’s scripture solely to Isaiah the prophet, respectively. The medieval commentator Rashi also attributed Isaiah’s scripture solely to Isaiah the prophet.
Such prophecies made German scholars in the 18th and 19th centuries uncomfortable since they realized their divine implications. They responded with the Deutero-Isaiah proposition, which suggests Isaiah was authored by numerous people over time and his books had divisions such as “Proto-Isaiah” (1-39), “Deutero-Isaiah” (40-55), and “Trito-Isaiah” (56-66). According to Ulrich Berges (2010), “The historical gap of more than 150 years which lies between Isaiah at the end of the 8th century and the time of the end of the exilic period presumed in Isaiah 40-55 (Cyrus’ decree in 539 B.C.E.), could, with the rise of the historical critical Bible interpretation, no longer be overcome merely by referring to the visionary power of Isaiah. To compound matters, Isaiah is said not only to have announced the prospect of salvation, but also to have mentioned the name of the new Persian ruler, Cyrus II (559-530) in Isa 44:28; 45:1. It was this problem which gave rise, toward the end of the 18th century, to the argument between ecclesiastical and rationalistic interpretation. his argument was not only concerned with the question as to which words can be traced back to Isaiah, but more fundamentally with the question as to what rationally comprehensible accreditation one was prepared to give to the prophets and what not. This is of utmost importance to the emergence of the Deutero-Isaiah hypothesis.”
The process of coming to a conclusion starts with one or more premises. When the premise is that prophecies are not possible, the conclusion is that Isaiah must have been written by someone else during the times of the prophesied events. Today’s scholars in many instances assume this conclusion and do not even source their first premise (e.g., Baltzer, 2010). They rather jump to the “scholarly consensus” of multiple authorship over time. See the sampling of scholars in my references for examples.
They do not claim to be in agreement on the specific dating of the supposed “redactions,” authorship attributions of sentences and chapters, location origin of the authorship (in Jerusalem, Babylon, or Persia), style variations, or unity or division of Isaiah, yet they seem to unite on the foundation, which is that he didn’t write his entire book (e.g., Baltzer, 2010; Becker, 2020; Berges, 2010; Buttenweiser, 1919; Coggins, 1998; Gitay, 1980; Goulder, 2004; Hurowitz, 2003; Mastnjak, 2020; and Niditch, 1980). At issue is they don’t have ancient support for their claims. Niditch (1980) even noted their lack of clear historical references and elusive support. In contrast, we have ancient attestations to support our claim that Isaiah authored Isaiah solely in the 8th century B.C.
Becker, U. (2020). The Book of Isaiah: its composition history. Oxford Handbooks Online. https://www.oxfordhandbooks.com/view/10.1093/oxfordhb/9780190669249.001.0001/oxfordhb-9780190669249-e-2#oxfordhb-9780190669249-e-2-note-64
Berges, U. (2010). The book of Isaiah as Isaiah’s book: the latest developments in the research of the prophets. Old Testament Essays, 23(3).
Buttenweiser, M. (1919). Where did Deutero-Isaiah live? Journal of Biblical Literature, 38(3/4), 94-112.
Coggins, R.J. (1998). Do we still need Deutero-Isaiah? Journal of the Study of the Old Testament, 81, 77-93.
Gitay, Y. (1980). Deutero-Isaiah: Oral or written? Journal of Biblical Literature, 99(2), 185-197. These arguments relate to those made by James Muilenburg (1956) Isaiah, Chapters 40-66, Introduction. IB 5.
Goulder, M. (2004). Deutero-Isaiah of Jerusalem. Journal for the Study of the Old Testament, 28(3), 351-362.
Hurowitz, V.A. (2003). Review: Restoring the Temple: Why and when? The Jerusalem Quarterly Review, 93(3/4), 581-591.
Josephus, The Life – Against Apion, with an English translation by H. St. J. Thackeray, Loeb Classical Library, London and New York, 1926, pp. 176–81. In Dunkelgrun, T. (2016). The Testimonium Flavianum Canonicum. Josephus as a witness to the Biblical canon. International Journal of the Classical Tradition, 23, 252-268.
Josephus, F. Antiquities of the Jews. English translation provided by William Whiston at Perseus at Tufts University: http://www.perseus.tufts.edu/hopper/text?doc=Perseus%3Atext%3A1999.01.0146%3Abook%3D11%3Awhiston+chapter%3D5%3Awhiston+section%3D2
Mastnjak, N. (2020). The book of Isaiah and the anthological genre. Hebrew Studies, 61, 49-72.
Rashi. Commentary on Isaiah 1:1. Sefaria. https://www.sefaria.org/Rashi_on_Isaiah.1.1.2?lang=bi