What was Isaiah’s sin?

Answer:

First, you need to understand that Isaiah is not necessarily in chronological order. As Adam Clarke notes:

"As this vision seems to contain a solemn designation of Isaiah to the prophetic office, it is by most interpreters thought to be the first in order of his prophecies. But this perhaps may not be so; for Isaiah is said, in the general title of his prophecies, to have prophesied in the time of Uzziah, whose acts, first and last, he wrote, 2Ch 26:22; which is usually done by a contemporary prophet; and the phrase, in the year that Uzziah died, probably means after the death of Uzziah; as the same phrase (Isa 14:28) means after the death of Ahaz. Not that Isaiah's prophecies are placed in exact order of time. Chapters ii., iii., iv., v., seem by internal marks to be antecedent to chap. i.; they suit the time of Uzziah, or the former part of Jotham's reign; whereas chap. i. can hardly be earlier than the last years of Jotham. See note on Isa 1:7, and Isa 2:1."

In other words, the prophecies are organized by subject more than they are organized by time. Isaiah was likely a prophet for God prior to the year Uzziah died, but in this chapter, he volunteers for greater and more difficult duties.

Isaiah's statement, "So I said: 'Woe is 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 my eyes have seen the King, the LORD of hosts'" (Isaiah 6:5), is a general statement of inadequacy. It is not one specific sin that he is confessing but that sin in his life and living among sinful people makes him unfit for the high calling that God has placed on him. It is no different than Peter's confession, "When Simon Peter saw it, he fell down at Jesus' knees, saying, 'Depart from me, for I am a sinful man, O Lord!'" (Luke 5:8). Moses also made a similar, though more subtle statement, "And Moses spoke before the LORD, saying, 'The children of Israel have not heeded me. How then shall Pharaoh heed me, for I am of uncircumcised lips?'" (Exodus 6:12). "Uncircumcised lips" directly parallels "unclean lips" in Isaiah's account. Both are saying they are unworthy to speak God's truths. In the presence of God Almighty, a person can only see his deficiencies. "I have heard of You by the hearing of the ear, but now my eye sees You. Therefore I abhor myself, and repent in dust and ashes" (Job 42:5-6).

"Then one of the seraphim flew to me, having in his hand a live coal which he had taken with the tongs from the altar. And he touched my mouth with it, and said: 'Behold, this has touched your lips; your iniquity is taken away, and your sin purged'" (Isaiah 6:6-7).

Again, the topic is not one specific sin, but that sin has made Isaiah realize that he was inadequate to serve the Holy God. "For all have sinned and fall short of the glory of God" (Romans 3:23). But God shows him that He is able to make him adequate for the task. It was done by taking a lump of live coal from the altar touching his lips with it. The altar was where the sacrifices were made on behalf of the sins of the people. God is saying that through sacrifice Isaiah would be freed from his sins; an allusion to the sacrifice of Christ, "And for this reason He is the Mediator of the new covenant, by means of death, for the redemption of the transgressions under the first covenant, that those who are called may receive the promise of the eternal inheritance" (Hebrews 9:15).