Charles Ellicott Commentary Isaiah 6:7

"and he touched my mouth with it, and said, Lo, this hath touched thy lips; and thine iniquity is taken away, and thy sin forgiven." — Isaiah 6:7 (ASV)

And he laid it upon my mouth. —So Jehovah touched the mouth of Isaiah’s great successor (Jeremiah 1:9); but not in that case with a coal from the altar. That prophet, like Moses (Exodus 4:10), had felt only or chiefly the lack of power (Alas! I cannot speak!), and power was given him. Isaiah desired purity, and his prayer also was answered.

Your iniquity is taken away, and your sin purged. —The clauses express the two elements of the great change which people, according to their varying systems, have called Conversion, the New Birth, Regeneration; but which is at all times a necessary stage in the perfecting of the saints of God. Pardon and purity are the conditions for both the prophet’s work and the completeness of his own spiritual life.