The hypothesis of modern critics is that Isaiah is not the sole author of the prophecy bearing his name, but that he only wrote chapters 1-39 (called by them "the former portion"), and that an unknown author or authors (for there are now alleged to have been three, or more, Isaiahs) are responsible for chapters 40 to the end (called by them "the latter portion").
  Thus, they would treat this prophecy much as Isaiah himself is said to have been treated, who, as tradition tells us, was "sawn asunder".
  This "latter portion" also modern critics would relegate to a later date: namely, toward the close of the seventy years' exile.
  This is a very modern theory; for, the one authorship of this prophecy has been held without question by both Jews and Christians for over 2,000 years.

I. THE USE OF HIS NAME IN THE NEW TESTAMENT.

  A sufficient and conclusive answer to this matter is afforded by Holy Scripture itself, in the fact that Isaiah is twenty-one times mentioned by name in the New Testament as the author of this prophecy.
  Eleven of these passages attribute to him words occurring in the latter portion of the book, and ten of them words occurring in the former portion.
  A complete list is appended, divided as follows:—

  (iv) And the prophet is named by seven different speakers or writers in the New Testament:

  Four times by Christ Himself; three being from the former portion of Isaiah (Matthew 13:14; 15:7. Mark 7:6), and one from the latter (Matthew 12:17).
  Twice by Matthew: once from the former portion (Matthew 4:14), and once from the latter portion (Matthew 8:17).
  Four times by Luke: all from the latter portion of Isaiah (Luke 3:4; 4:17. Acts 8:28; 8:30).
  Three times by John the Evangelist: twice from the former portion (John 12:39, 41), and once from the latter portion (John 12:38).
  Twice by John the Baptist: both from the latter portion (Matthew 3:3. John 1:23).
  Six times by Paul the Apostle: four from the former portion (Acts 28:25. Romans 9:27, 29; 15:12), and twice from the latter portion (Romans 10:16, 20).

II. THE EMPLOYMENT OF CERTAIN WORDS.

  A further evidence of the unity of Isaiah is furnished by the Structure of the book: which, as the student of The Companion Bible will readily perceive, does not lend itself in any degree to the arbitrary ending suggested, at chapter 39.

  A "pillar" of this "theory" is found in the supposed occurrence of certain words in the "former" portion of the prophecy which are not found in the "latter" portion, and vice versa. An examination of a few such words which are cited by modern critics will show the palpable inaccuracy characterizing their assertions.

  It is asserted that the following are found only in the "latter" portion of Isaiah (chapters 40 to the end):—

1. The titles Creator, Redeemer, Savior. But the facts of creating, redeeming, and saving are referred to in 1:27; 12:1, 2; 14:1; 17:10; 25:9; 27:11; 29:22; 30:18; 33:22; 35:10.
2. The thought of Jehovah as "Father". But the relation is stated in 1:2.
3. The word bachar (to choose). But see 1:29; 7:15, 16; 14:1.
4. The word halal (to praise). But see 13:10; 38:18.
5. The word paer (to glorify). But see 10:15.
6. The word patsach (to break forth into joy). But see 14:7.
7. The word tsemach (to spring forth). But see 4:2.
8. The word zero' (the arm [of Jehovah]). But see 9:20; 17:5; 30:30; 33:2.

  There are more than 300 words and expressions which are common to both the alleged "former" and "latter" portions of Isaiah's prophecy; and which do not occur at all in the later prophecies of Daniel, Haggai, Zechariah, and Malachi.

  A sufficient number of these, to illustrate this fact amply, will be found given in the notes under their occurrences.