Not a bad film, but to try to link it to the Omen Triology was very poorly done. If the Catholic Church wanted to control the coming of the Antichrist why give Damien away out of their control to the American ambassador?
In the original Omen film Damien is born of a jackel. That was shown in the graveyard screen when the Ambassador opens Damien's mother's grave.
In the First Omen, the final scene is the new born children being swapped. In the original film the child born to the Ambassador & his wife was stillborn & he 'adopted' a baby from a monastery/nunnery in place of his child, to avoid his wife's grief.
I guess the scriptwriters didn't watch the original film or couldn't work out a better link. Very disappointing.