Few scholars hold the traditional view that Isaiah wrote the entire book. The majority acknowledges that the prophet wrote chapters 1–39, and from there, disagreement exists as to how many other authors or sections make up the book. Chapters 40–66 are often agreed to have had one or more authors. The language of chapters 40–55 reads like after the exile, and the remaining chapters read like returning from exile. These two sections are respectively referred to as Deutero-Isaiah and Trito-Isaiah. Some believe there are two divisions, while others hypothesize there are more than three divisions. Indeed, authorship would not have been a concern to earlier audiences, and it’s only a topic due to a scientific method of interpreting literary texts alike—a combination of higher and lower criticism. Scripture, therefore, is reduced to academic work and not as highly esteemed as divinely inspired.
As far back as 1100 CE, Moses ben Samuel denied Isaiah was the author of specific chapters. A few years later, Ibn Ezra (ca. 1167) questioned certain sections as being from the hand of Isaiah. Finally, in the 18th century, Johann Doerderlein suggested that Isaiah could not have foreseen the fall of Jerusalem, the 70-year captivity, the return, or the Persian king, Cyrus. Since his thesis, scholarship has advanced in supporting dual or multiple authorship. This is readily accepted in academic circles and in some churches too.
From chapters thirty-nine to forty, we skip ahead about 150 years. The opening passages of chapter forty suggest Israel’s sins have been paid for and that they need to be comforted (40:1–2). When we left off in chapter thirty-nine, Hezekiah had survived the Assyrian assault and sickness that threatened to take his life. Isaiah told him that Jerusalem’s destruction would not happen in his lifetime, concluding eighth-century Isaiah. Jerusalem fell in 586 BCE, and exiles returned after 538 BCE. Isaiah 42:24–25 refers to the destruction of Jerusalem as a past event, but this isn’t altogether a reason to believe in multi-authored Isaiah. The early chapters of Isaiah speak of things as both present and past—especially the first five chapters, which are not wholly chronological. However, Isaiah 43:14 reads as if the Jews were in Babylon, and Cyrus is explicitly named in 44:28. When we arrive at Isaiah 66:20, the second temple is under construction. The language of multiple timelines is a big reason for the thesis when placed alongside the other mentioned details. Plus, after chapter thirty-nine, Isaiah isn’t mentioned again.
The belief in multiple authors has some merit, but it denies the prophet’s ability to speak about and predict future events. The German schools of biblical studies rely heavily on this scientific method of interpretation, contributing to scholarship significantly. Still, it depends upon naturalism without leaving room for divine inspiration or the miraculous. This was, in part, a result of David Hume’s Enlightenment philosophy which grew from Newtonian reasoning. He believed that a miracle was a violation of the laws of nature, as he so wrote in Of Miracles in An Inquiry Concerning Human Understanding, so they simply did not exist. This would remove the ability of a prophet of God to be moved by God’s Spirit to talk about future events with the voice of God, thus limiting God. Hume would write, “It is experience only, which gives authority to human testimony; and it is the same experiences, which assures us of the laws of nature.” Because he and most of his peers never “experienced” such a phenomenon, the divine is reduced to naturalism. Philosopher Peter Kreeft observed, “In fact, all the essential and distinctive elements of Christianity are miracles: creation, revelation (first to the Jews), the giving of law, prophecies, the Incarnation, the Resurrection, the Ascension and the Second Coming and Last Judgment.”
The cases of multiple authors can be convincing, but I hold to the traditional view that Isaiah is the primary source of the book bearing his name (cf. John 12:37–38; Is. 53:1). The oldest extant copy of Isaiah dates to 175 BCE and is a single scroll without any notes from copyists about a shift from chapters 39–40. Archaeologists found twenty-one copies of Isaiah among the Dead Sea Scrolls and six manuscripts of commentaries on Isaiah. Isaiah has also been referenced as the only author of the book. It is the most quoted book in the New Testament, with Isaiah as only ever cited as the author and the quotes attributed to him from the various sections of texts that scholars dispute (cf. Matt. 3:3; 4:14; 8:17; 12:17; et al.).
I agree with John Oswalt, professor of Old Testament at Asbury Seminary, that the entirety of the book was assembled over the years by Isaiah’s disciples from his speeches, remarks, and other verbal communication. Isaiah, therefore, is the source of the entire book, but its compilation was likely the work of disciples. Just a few centuries before Isaiah, we read about schools of prophets (1 Sam. 10:5, 10; 19:20). In the first passage from Samuel, the Targum (Aramaic translation of the Old Testament dating to 516 BCE) uses the phrase “band of students.” In the second passage, Samuel is the leader of the band of prophets. Elijah and Elisha exhibited the disciple-teacher relationship within a prophetic background (2 Kings 2:3). Elisha served Elijah (2 Kings 3:11) and was anointed by him (1 Kings 19:16). Isaiah and his disciples and learned ones may be another example of this relationship (Is. 8:16; 50:4). Given the notion of destruction and exile in the earlier chapters of Isaiah, it makes sense that he would have spoken to future generations. Later, his disciples may have redacted portions of his speeches and writings to fit the times, which scribes often did (compare Gen. 14:14 to Judg. 18:12). We also can’t discount the usage of dictation to scribes, which biblical authors often employed (cf. Jer. 36:4; Rom. 16:22), when accounting for variations in syntax.
The matter isn’t a hill to die on but another perspective to the recently formed (18th century) and common thesis. Some might contend that the evidence of Cyrus’ name is compelling, but the name may have been inserted by later scribes based on the description provided. The same may be said of other portions that lead readers to conclude multiple authors regarding Deutero- and Trito-Isaiah. Scribes could have changed the language to show the fulfillment of specific prophecies. We don’t know. The only copies we have are from the second century BCE and later, and a study published in 2021 concluded that two scribes wrote the Great Isaiah Scroll. The final form of Isaiah may have emerged in the post-exilic period. Still, it doesn’t mean another version may have predated what we have that’s original to the eighth-century prophet.