Origen
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.
But if we would rise to a more hidden meaning, I should say, that there were certain shepherd angels, who direct the affairs of men, and while each one of them was keeping his watch, an angel came at the birth of the Lord, and announced to the shepherds that the true Shepherd had arisen. For Angels before the coming of the Saviour could bring little help to those entrusted to them, for scarcely did one single Gentile believe in God. But now whole nations come to the faith of Jesus.