The Book of Mormon clarifies these two difficult verses. It teaches that the people to whom Isaiah was sent would fail to understand his message of their own accord. The KJV implies that the Lord did not want them to understand (v. 9).
9 And he said: Go and tell this people— Hear ye indeed, but they understood not; and see ye indeed, but they perceived not.9 And he said, Go, and tell this people, Hear ye indeed, but understand not; and see ye indeed, but perceive not.10 Make the heart of this people fat, and make their ears heavy, and shut their eyes— lest they see with their eyes, and hear with their ears, and be converted and be healed. [2 Nephi 16:9–10]10 Make the heart of this people fat, and make their ears heavy, and shut their eyes; lest they see with their eyes, and hear with their ears, and understand with their heart, and convert, and be healed. [Isaiah 6:0–10]
The “making fat” of the hearts of the people (v. 10) seems to refer to Isaiah’s being called to make the truth so plain that they would either have to accept it or harden their hearts against it. According to Brigham Young and Willard Richards this was how the Lord hardened Pharaoh’s heart in Egypt:
God has promised to bring the house of Israel up out of the land of Egypt at his own appointed time; and with a mighty hand and an outstretched arm and great terribleness (Deut. 26:8). He chose to do these things that His power might be known and his name declared throughout all the earth, so that all nations might have the God of heaven in remembrance, and reverence His holy name; and to accomplish this it was needful that He should meet with opposition to give Him an opportunity to manifest His power; therefore He raised up a man, even Pharaoh, who, He foreknew, would harden his heart against God of his own free will and choice, and would withstand the Almighty in His attempt to deliver His chosen people, and that to the utmost of his ability; and he proved himself worthy of his choice, for he left no means unimproved which his wicked heart could devise to vex the sons of Abraham, and defeat the purposes of the Most High, which gave the God of Abraham the opportunity to magnify his name in the ears of the nations, and in sight of this wicked king, by many mighty signs and wonders, sometimes even to the convincing of the wicked king of his wickedness, and of the power of God (Exodus 8:28, etc.), and yet he would continue to rebel and hold the Israelites in bondage; and this is what is meant by God’s hardening Pharaoh’s heart. He manifested Himself in so many glorious and mighty ways, that Pharaoh could not resist the truth without becoming harder; so that at last, in his madness, to stay the people of God, he rushed his hosts into the Red Sea and they were covered with the floods. [HC, 4:263–264]
The last half of Isaiah 6:10 may be misread and lead to the conclusion that the Lord does not want the people to be converted and healed. The real meaning of the last part of the verse, as it is fully quoted in the New Testament, is a declaration that the people did not want to understand, lest they should be converted so that the Lord could heal them (see Matthew 13:14–15 and Acts 28:26–27).
In other words, these people did not want to know the truth. They enjoyed living in sin, and did not want to be converted and then have to change their life-styles.