“Christians refused to . . . sacrifice to the emperor’s genius—roughly equivalent today to refusing to salute the flag or repeat the oath of allegiance.

“The act of emperor worship consisted in sprinkling a few grains of incense or a few drops of wine on an altar which stood before an image of the emperor. Perhaps at our long remove from the situation we see in the act nothing different from . . . lifting the hand in salute to the flag or to some distinguished ruler of state, an expression of courtesy, respect, and patriotism.