The purpose of this article is to show that there is but one inspired penman of the book of Isaiah. Ultimately the author of all of the canonical books, both Old and New Testament, is God Himself through the Holy Spirit.
“God, who at sundry times and in divers manners spake in time past unto the fathers by the prophets.” — Hebrews 1:1
The Bible teaches us that all scripture is by inspiration of God. 2 Timothy 3:15, “All scripture is given by inspiration of God…” So we can see here that God is the ultimate author of the Bible including the book of Isaiah. One must keep this principle in mind when considering the authorship of any book in the Holy Writ.
Throughout the first 2500 or so years since Isaiah first began to prophesy there has been little opposition to the fact that the entire book, all 66 chapters, was written by the same Isaiah, who was the son of Amoz, as introduced in Isaiah 1:1. For more than two-and-a-half millennia, it was regarded by all Christians and Jews that this same man, Isaiah, was an inspired man of God and was the sole human penman of the book.
The Arguments for Multiple “Isaiahs”
Beginning in the late 1700s, the critical school of theology began to tear down the authorship of Isaiah. They hold the view, contrary to 2 Timothy 3:15, that all scripture is not inspired by God. Their teachings are that the human writers wrote what they thought to be true. The prophets are said to be mere humans who have no direct influence by the Holy Spirit. The “Higher Critics” claim the prophets, especially Isaiah, could not have possibly predicted things hundreds of years in the future. They rationalize that Isaiah only wrote the prophecies that were fulfilled in his lifetime, after the fact of course. They claim that others added their works to Isaiah’s book, which has, to them become simply an anthology of prophetic writings.
The critical disintegration of the book began with Koppe, who in 1780 first doubted the genuineness of Isaiah 50. Nine years later Doederlein suspected the whole of Isaiah 40 through 66. He was followed by Rosenmueller, who was the first to deny to Isaiah the prophecy against Babylon in 13:1 through 14:23. Eichhorn, at the beginning of the last century, further eliminated the oracle against Tyre in Isaiah 23, and he, with Gesenius and Ewald, also denied the Isaianic origin of Isaiah 24 through 27. Gesenius also ascribed to some unknown prophets Isaiah 15 and 16. Rosenmueller then went farther and pronounced against Isaiah 34 and 35, and not long afterward (1840) Ewald questioned Isaiah 12 and 33. Thus by the middle of the 19th century, some 37 or 38 chapters were rejected as no part of Isaiah’s actual writings. 
The critics maintained for the next 100 years or so that the writer of chapters 40-66 was “Deutero-Isiah.” This anonymous writer is said to have lived during the Babylonian captivity and is credited for the Messianic prophecies in the book. They made this claim of Deutero-Isaiah based upon the difference in style between the first and second sections of the book. They hold this position because of their unbelief of the supernatural nature of scripture. Near the end of the 19th century, others came along and denied the writings could have all been written by both Isaiah and Deutero-Isaiah.
What, prior to 1890, was supposed to be the unique product of some celebrated but anonymous seer who lived in Babylonia about 550 bc is today commonly divided and subdivided and in large part distributed among various writers from Cyrus to Simon (538-164 bc). At first, it was thought sufficient to separate Isaiah 63 through 66 as a later addition to “Deutero-Isaiah’s” prophecies; but more recently it has become the fashion to distinguish between Isaiah 40 through 55, which are claimed to have been written by “Deutero-Isaiah” in Babylonia about 549-538 bc, and Isaiah 56 through 66, which are now alleged to have been composed by a “Trito-Isaiah” about 460-445 bc. 
By the early 1900s, the critical views went as far as to “throw out” 44 of the 66 chapters or approximately 800 of the 1292 verses. These are the numbers accepted by those who were considered moderates. Those who were considered to hold a radical view “threw out” approximately 1,030 verses out of the total 1,292. These critics reject all but 262 verses of the book of Isaiah as genuine writings of Isaiah.  One can see that the critical view of the authorship of Isaiah is quite confusing.
The Argument for a Single Isaiah
The critical view is not a scriptural view of authorship. The New Testament scriptures support the fact that all 66 chapters and all 1292 verses were written by a single prophet, Isaiah. Isaiah’s writings are the most often cited by the writers of the New Testament.
In the New Testament, Isaiah is quoted more than all the other prophets together, and this is done in such a way as to leave no room for doubt that, in the eyes of the New Testament, Isaiah was the author of the entire prophecy. 
The New Testament writers are all in agreement that there is only one author of the book. The different sections of Isaiah that are attributed to the multiple authors by the modern critics are all quoted from in the New Testament as being authored by Isaiah.
When quoting from those sections of Isaiah that the modernists have attributed to someone else other than Isaiah, the New Testament writers always assign them to Isaiah. Within John 12:38-41, quotations are found from both parts of Isaiah, and they are assigned to Isaiah. Paul, in Romans chapters nine through eleven, cites Isaiah as the author of the entire book that bears his name. 
The modern critics of the individual authorship of Isaiah hold in doubt the verbal, plenary inspiration of the Bible. By denying the single authorship of Isaiah, they cast dispersions on the New Testament writers who uphold Isaiah’s authorship. Who is right? The modern, uninspired critics or the ancient, inspired holy men of God.
This one argument based on the testimony of the inspired New Testament writers should be sufficient to prove the authorship of Isaiah. But the “Higher Critics” of today seem to believe that there must first be secular or extra-biblical evidence supporting biblical assertations before they will believe any part of it. A reliable secular source of the late first century is Josephus. He states in his book Antiquities of the Jews that Cyrus, king of Persia “read about himself in the prophecies of Isaiah.” Josephus showed that during the time of Cyrus and during his own time that Isaiah was believed to be the author of the prophecies concerning Cyrus. It is interesting to note that those prophesies were made more than 200 years before Cyrus. Cyrus read those prophecies in the book of Isaiah 600 years before Christ and also before the so-called “Deutero-Isaiah” is said to have written his addition (chapters 40-66.)
Another strong argument for the one author of Isaiah came to light after the 1947 discovery of the Dead Sea Scrolls. One of the most important writings found in the caves at Qumran was a scroll containing a nearly complete copy of the book of Isaiah. This copy contains only 39 differences from later, more modern copies of Isaiah, and these are mainly grammatical and spelling differences. What is interesting, for the purpose of dispelling, the critical argument, is that the sections they suppose to be written by different authors are not broken up in the “Dead Sea” Isaiah scroll. In the scroll, chapter 40, begins immediately after the last word of chapter 39 with no space in between.  Apparently, the scribe who copied that scroll did not realize that chapters 39 and 40 were “written by two different Isaiah’s.”
Implications of Multiple Authorship of Isaiah
Denying Isaiah’s authorship of the entire prophecy denies the verbal, plenary inspiration of the scriptures. The critics refuse to believe one man could have such accurate predictions of the future. They refuse to give credit to the Holy Spirit for inspiration. They make the assertation that the Bible is written by men based upon oral traditions rather than by God and verbal inspiration through the Holy Spirit.
In claiming that there were multiple writers of Isaiah, in the face of the evidence given by the New Testament writers, to the contrary, they attempt to discredit these New Testament writers. If one cannot rely on the New Testament writers to correctly assert the truth concerning the authorship of the book of Isaiah, how can one rely on them, to tell the truth concerning the Gospel of Jesus Christ?
by Jack W. McNiel Credits:  Orr, James; International Standard Bible Encyclopedia, (Electronic Edition STEP Files Copyright © 1998, Parsons Technology, Inc.) Isaiah 9.1  Ibid. Isaiah 9.2  Ibid. Isaiah 9.4  Young, Edward; An Introduction to the Old Testament, (William B. Eerdmans Publishing, 1964) p. 205  Brown, David; Isaiah Volume I: Modernism’s Attack on Isaiah, (Bible Resource Publications, 1995) p. 16  Ayers, Tim; Southwest School of Biblical Studies, Class Notes: Introduction to Isaiah. P.2  Ibid. p.2.