CLAIM: Isaiah doesn’t get his calling until chapter six. Critics argue that this demonstrates that a redactor either added the first five chapters, or he inserted chapter six. Is this the case? Why else would Isaiah place his calling this far into the book?
RESPONSE: Before Isaiah explained his calling, he wanted to show the depravity of Israel and the dire circumstances that led him to come to God. Without this historical backdrop, Isaiah’s calling wouldn’t have made as much sense. When we understand the sinfulness of Israel and the sinfulness of Isaiah himself (Isa. 6:5), it makes God’s calling all the more gracious and incredible.
Consider an action movie about a vigilante hero, who is trying to rescue his city from mobsters. The movie might open with several scenes about the depravity and cruelty of the mobsters and the indifference of the city, before the hero is introduced. When the hero finally comes on screen, this would demonstrate to the audience how brave this man is to stand up to violence and tyranny. In the same way, Isaiah’s calling is even more miraculous, when we consider how fallen Israel was in chapters 1-5. Moreover, this story demonstrates that if Isaiah can be redeemed (a man of unclean lips—verse 5), then so can the nation of Israel. We see this same motif in Amos, who is not introduced until Amos 7:10-17. Grogan comments, “Our sense of difficulty may well reflect modern Western individualism, for the Bible teaches that the message is far more important than the messenger (Phil 1:15–18).” In other words, the book of Isaiah is not about Isaiah. Instead, the book is about God’s message that he brought through Isaiah.
 Grogan, G. W. (1986). Isaiah. In F. E. Gaebelein (Ed.), The Expositor’s Bible Commentary: Isaiah, Jeremiah, Lamentations, Ezekiel (Vol. 6, p. 54). Grand Rapids, MI: Zondervan Publishing House.