Factual error: In the cemetery, the skeleton of Gregory Peck's son is far too big to be that of a newborn, which it is supposed to be.
Factual error: Jennings says that the name of Megiddo derives from Armageddon and that it is south of Jerusalem. In actual fact, Megiddo is the source of the word Armageddon, not the other way around, and it is a long way north of Jerusalem. (01:26:30)
Factual error: It is highly unlikely that a British police detective making routine enquiries would be carrying a gun.
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.
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)
Answer: It's possible that the mark didn't appear until Mrs Baylock showed up and began teaching him about his heritage. The mark of the devil is said to appear only when Satan accepts a follower, and up until Mrs Baylock appears, Damien is a normal boy. Yes, he's the devil's son, but he knows nothing of his evil nature until she starts teaching him. In fact, at one point, Robert mentions that he knew every inch of Damien's body since he raised him and never saw anything, so more than likely the mark didn't appear until Damien had areas on his body where it could be hidden more easily.
dewinela
But it's a birthmark. He had it when he was born.