Mrs Baylock: Have no fear, little one... I am here to protect thee.
Damien: Please, Daddy! No, Daddy, no.
Father Brennan: He must DIE, Mr. Thorn.
Father Brennan: When the Jews return to Zion / And a comet rips the sky / And the Holy Roman Empire rises, / Then You and I must die. / From the eternal sea he rises, / Creating armies on either shore, / Turning man against his brother / 'Til man exists no more.
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