Factual error: In the opening scene, Gregory Peck is riding in the dark in a car in Rome on June 6th at 6:00am. The sun rises at 5:35am in Rome on June 6th.
Continuity mistake: When Katherine falls off the 2nd floor and lands her right arm is pointing up, but in the next shot her right arm is pointing down.
Factual error: Keith Jennings refers to an astrologers' publication which describes a star "like the star of Bethlehem" but "on the other side of the world, the European continent." Bethlehem is not on the other side of the world from Europe, it's at the same longitude as eastern Europe and less than 800 miles from the European mainland. (01:00:20)
Chosen answer: The idea wasn't entirely Hollywood's. The antichrist is sometimes said to be "born of a jackal. " Jackals are mentioned in the Bible, and some interpretations consider them a symbol of something that is "not of God." In the movie, it means that the kid was scary as hell, literally.
Hobbes