Gregory the Great, on Ps 103:4

“Who makest thy angels spirits: and thy ministers a burning fire.”

“It should also be known that the word "angel" is a name of office, not of nature. For those holy spirits of the heavenly homeland are indeed always spirits, but they cannot always be called angels, since they are angels only when something is announced through them; hence it is said through the Psalmist: "Who makes His angels spirits." As if he were saying plainly: He who always has them as spirits also makes them angels when He wills. Moreover, those who announce lesser things are called angels, while those who announce the greatest things are called archangels. This is why not just any angel, but the archangel Gabriel, was sent to the Virgin Mary. For it was fitting that the highest angel should come for this ministry, since he was announcing the highest of all things.”

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