The angel of the Lord came upon them; better, an angel. The Greek word rendered "came upon them"—a very favorite word with St. Luke—suggests a sudden appearance. The glory of the Lord shone round about them: and they were sore afraid.
The white shining cloud of intolerable brightness, known among the Jews as the Shechinah, the visible token of the presence of the Eternal, in the bush, in the pillar of fire and cloud which guided the desert-wanderings, in the tabernacle and the temple.
It shone round the Redeemer on the Mount of Transfiguration. It robed him when, risen, he appeared to the Pharisee Saul outside Damascus. The occasional presence of this visible glory was exceedingly precious to the chosen people.
The terror felt by the shepherds was the natural awe ever felt by man when brought into visible communion with the dwellers in the so-called spirit-world.