Isaiah stated, "Wo is unto me! for I am undone; because I am a man of unclean lips; and I dwell in the midst of a people of unclean lips; for mine eyes have seen the King, the Lord of Hosts." Unclean lips! That was the first thing that Isaiah felt self-conscious about in the presence of God. He was a prophet. His main tool was speech. Isaiah spoke, and yet felt that his voice was unclean or inadequate. It is an interesting warning that even though we do all the right things and we go through the right motions, if our lips and our thoughts are not pure, we will strongly feel shame in the presence of God. We need to be clean—every whit.
Isaiah’s concern with his unclean lips is then addressed: "Then flew one of the seraphim unto me, having a live coal in his hand." This coal was some kind of a burning ember. It may have been a piece of burning wood. I doubt that they had charcoal briquettes or even coal. It may have been hot ashes. Nonetheless, the seraph had something very hot in his hand, "which he had taken with the tongs from off the altar; And he laid it upon my mouth, and said: Lo, this has touched thy lips; and thine iniquity is taken away, and thy sin purged."
How did people living in ancient times determine whether or not a witness was telling the truth? How could they know whether to rely on the veracity or truthfulness of the testimony of a witness? We know from some ancient Near Eastern texts that "trial by ordeal" was used in situations where it was difficult to determine if a witness was speaking truth during his testimony at trial. Any person who strongly questioned whether a particular witness was telling the truth could challenge that witness and force him to submit to a specified "ordeal." Something very hot (a hot spatula or coal) was put on the tongue of the witness. If the witness was hurt or injured during the ordeal, it was considered to be evidence that he was lying. The reasoning was that someone who is lying would have a dry mouth and tongue. Therefore, a liar would not be able to tolerate something hot placed on his tongue. It would hurt a lot and cause physical damage. However, if the witness was telling the truth, he would be relaxed and his tongue would be coated with saliva to prevent the heat from searing and hurting his tongue. This operated like a type of primitive lie detector test.
I wonder if Isaiah saw himself being subjected to this type of ordeal. He passed the test. Isaiah was sanctified by the Holy Ghost, and his sins were forgiven. This is one of the responsibilities of the Holy Ghost, is it not?
This experience was symbolic. I do not believe Heavenly Father was chastising Isaiah with a negative message like, "You bad, evil, child. Let me cleanse you." I think Heavenly Father was comforting Isaiah and addressing his concerns or doubts about his ability to preach repentance to the people in Jerusalem. The Lord was telling Isaiah that he was forgiven of his sins and that he would receive divine assistance in his personal ordeals as he preached truth. It reminds me of Enoch’s doubts when he was called as prophet. Enoch knew that he was slow of speech and questioned why he would be called as a prophet. We all feel inadequate about these types of callings, and Isaiah was no exception.
Jeffrey M. Bradshaw and David J. Larsen, In God’s Image and Likeness 2: Enoch, Noah, and the Tower of Babel (Salt Lake City, UT: The Interpreter Foundation and Eborn Books, 2014), 36: "Obvious similarities with the calls of Moses and Jeremiah present themselves in this verse. Moses responds to his call as follows: ‘Who am I, that I should go unto Pharaoh, and that I should bring forth the children of Israel out of Egypt?’ Later Moses objects more specifically in saying that he was ‘slow of speech, and of a slow tongue.’ Jeremiah complains by saying: ‘Ah, Lord God! behold, I cannot speak: for I am a child.’ Enoch combines the objections of Moses and Jeremiah, adding that ‘all the people hate me.’"
Blake T. Ostler, "The Throne-Theophany and Prophetic Commission in 1 Nephi: A Form-Critical Analysis," BYU Studies 26 no. 4 (1986): 72, 83.