[ I composed this some time ago, and just realized I never published it. In this brief study I set forth my understanding of the authorship of Isaiah's prophecy.]Time frame of Isaiah’s ministry:
The vision of Isaiah the son of Amoz, which he saw concerning Judah and Jerusalem in the days of Uzziah, Jotham, Ahaz, and Hezekiah, kings of Judah. (Isaiah 1:1)
Isaiah’s call during the reign of Uzziah (Isaiah 6:1-3)
In the year that King Uzziah died I saw the Lord sitting upon a throne, high and lifted up; and the train of his robe filled the temple. 2 Above him stood the seraphim. Each had six wings: with two he covered his face, and with two he covered his feet, and with two he flew. 3 And one called to another and said: “Holy, holy, holy is the Lord of hosts; the whole earth is full of his glory!”
The Length of each King’s reign:
In the twenty-seventh year of Jeroboam king of Israel, Azariah the son of Amaziah, king of Judah, began to reign. 2 He was sixteen years old when he began to reign, and he reigned fifty-two years in Jerusalem. (2 Kings 15:1-2a)
In the second year of Pekah the son of Remaliah, king of Israel, Jotham the son of Uzziah, king of Judah, began to reign. 33 He was twenty-five years old when he began to reign, and he reigned sixteen years in Jerusalem. (2 Kings 15:32-33a)
In the seventeenth year of Pekah the son of Remaliah, Ahaz the son of Jotham, king of Judah, began to reign. 2 Ahaz was twenty years old when he began to reign, and he reigned sixteen years in Jerusalem. ( Kings 16:1-2a)
In the third year of Hoshea son of Elah, king of Israel, Hezekiah the son of Ahaz, king of Judah, began to reign. 2 He was twenty-five years old when he began to reign, and he reigned twenty-nine years in Jerusalem. (2 Kings 18:1-2a)
The last historical mention of Isaiah is in the 14th year of Hezekiah’s reign as set forth in 2 Kings 18:13 - 20:21 and restated in Isaiah 36-39. Apocryphal tradition says that Isaiah was martyred by Hezekiah’s wicked son Mannasseh.
Doing the math for the length of Isaiah’s ministry, by the time of Hezekiah’s 14th year, we come up with a total of 47 years of prophetic ministry up to that point. We do not know how old Isaiah was when he received his call to the prophetic ministry in that last year of Uzziah’s reign. That said, it is reasonable to think that by the 14th year of Hezekiah’s rule, Isaiah was very probably well over 60 some years old.
My hypothesis is that Isaiah had already written much of the first 39 chapters before that time of Hezekiah’s reign and finished up those chapters shortly after that point. Chapters 40-66 were then written in the remaining 15 years of Hezekiah’s reign. In other words the bulk of Isaiah 1-39 was written over the course of those 47 years of active ministry, and chapters 40-66 were the result of the Holy Spirit using Isaiah’s theological maturity and meditative reflection in his “retirement” years. There were “two Isaiah’s” but they were not different “Isaiahs”; they were the same person at different periods in his life.
We are given good reason in Isaiah 39 to believe those last fifteen years of Hezekiah’s reign and life were relatively peaceful and tranquil. We read in verse 8 that Hezekiah said to Isaiah, “The word of the Lord that you have spoken is good.” For he thought, “There will be peace and security in my days.”