Continuity mistake: In the first two Omen movies, Damien's mark of the Beast was hidden on the top of his scalp. In this movie, it's behind his right ear.
Character mistake: Since the Book of Revelation ends with Satan being defeated, Damien and his followers should be aware of this. That's why it makes no sense that they keep quoting from Revelation since it ends up spelling their demise (it does not predict that the Anti-Christ will destroy the earth as they keep claiming).
Answer: Damien states at one point that as the birth of the Nazarene gets closer, his strength fades accordingly. Presumably this also applies to any forces assisting him.