The hot coal taken from the altar and placed on Isaiah's lips is symbolic of the atoning sacrifice of the Only-Begotten. There is no other way that iniquity can be taken away or sin purged. As explained in Enos,
'And there came a voice unto me, saying: Enos, thy sins are forgiven thee, and thou shalt be blessed.
And I, Enos, knew that God could not lie; wherefore, my guilt was swept away.
And I said: Lord, how is it done?
And he said unto me: Because of thy faith in Christ, whom thou hast never before heard nor seen. And many years pass away before he shall manifest himself in the flesh; wherefore, go to, thy faith hath made thee whole.' (Enos 1:5-8)
Isaiah doesn't ask, "how is it done?" But the next subject of discussion is 'Whom shall I send (to take away the sins of the world)?'