Gregory the Great — as excerpted in the Catena Aurea on Luke 2:8-12

“(ubi sup.) It was in a mystery that the Angel appeared to the shepherds while they were watching, and the glory of the Lord shone round about them, implying that they are thought worthy above the rest to see sublime things who take a watchful care of their faithful flocks; and while they themselves are piously watching over them, the Divine grace shines widely round about them.”