Gregory the Great, on Isa 6:7

“And he touched my mouth, and said: Behold this hath touched thy lips, and thy iniquities shall be taken away, and thy sin shall be cleansed.”

“But Dionysius the Areopagite, that ancient and venerable Father, is said to declare that from the lesser hosts of angels some are sent forth to fulfill their ministry either visibly or invisibly, namely because angels or archangels come for human consolation. For those higher hosts never withdraw from the inner places, since those who are preeminent have no function of external ministry. This seems to be contradicted by what Isaiah says: "And one of the Seraphim flew to me, and in his hand was a coal which he had taken with tongs from the altar, and he touched my mouth." But in this statement of the prophet, he wants it understood that the spirits who are sent receive the name of those whose office they perform. For the angel who carries a coal from the altar to burn up the sins of speech is called a Seraphim, which means burning.”

Imported from an open dataset — not yet checked against the printed edition.