Gregory the Great
Catena Aurea · Lukasevangelium 2
And there were in the same country shepherds abiding in the field, keeping watch over their flock by night. And, lo, the angel of the Lord came upon them, and the glory of the Lord shone round about them: and they were sore afraid. And the angel said unto them, Fear not: for, behold, I bring you good tidings of great joy, which shall be to all people. For unto you is born this day in the city of David a Saviour, which is Christ the Lord. And this shall be a sign unto you; Ye shall find the babe wrapped in swaddling clothes, lying in a manger.
(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.