The quotation in verses 2 and 3 is composite, the first part being from Mal 3:1 and the second part from Is 40:3. It is easy to see, therefore, why copyists would have altered the words ...
Whatever obstacle there existed to your communicating the message of God to this people, arising from your own consciousness of unworthiness, is taken away. You are commissioned to bear that message, and your own consciousness of guilt should not be a hindrance.
The Hebrew-English Old Testament Interlinear (HEOTI), Produced by Christian Publishing House, Cambridge, Ohio
Bible critics would tell us that the book of Isaiah is like a tapestry, with many hands contributing to its greater unity. Scholars recognize at least three distinct authors in the text. Is this true?
For about twenty-five centuries no one dreamt of doubting that Isaiah the son of Amoz was the author of every part of the book that goes under his name; and those who still maintain the unity of authorship are accustomed to point, with satisfaction, to the unanimity of the Christian Church on the matter, till a few German scholars arose, about a century ago, and called in question the unity of this book. What is true?
Some have looked to the style throughout the book of Isaiah and have suggested two Isaiah’s, a “Second Isaiah,” “the idea of a multiple authorship of Isaiah has arisen only in the last two centuries. Its simplest, most persuasive form is the ascription of chapters. 1–39 to Isaiah and 40–66 to an anonymous prophet living among the sixth-century exiles in Babylonia.”