Question: Why does Satan go through the trouble of hiring protective detail at the beginning of the film? Bullets have no effect on him, and even if he was hit, he could just make the claim that the shooter missed. And it's not like he's overly worried about people discovering his true identity, since he blackmails and corrupts people throughout the movie.
Question: Why exactly does the devil need to have a child to conquer the world? Why can't he do it himself? He can easily corrupt mankind and lead them to their destruction, which could allow him to take over the world.
Answer: According to the bible the devil has no power on earth, but like God, he wanted to send his son into the world to influence and corrupt them.
Question: Why exactly did the Devil keep Jericho alive? Why didn't he just have his followers kill him while they were tying him up?
Answer: It is never explained and is considered by many to be a major flaw in the film. Presumably Satan wanted to torture Jericho, he definitely seemed to enjoy tormenting him throughout the film.
Answer: It's not explained why he didn't kill him then. Any answer is speculative, but the obvious one is that Jericho is the main protagonist in the story and the plot needs to be allowed to play out in a contrived way to provide a satisfying conclusion. Abruptly killing off the character would end the story.
Question: How was Thomas Aquianes able to track down Christine in New York city? He was never told where she was born and for all he knew, she could've been born anywhere?
Answer: The Vatican is aware of the birth, somewhere in the world, of Christine (though not knowing her identity at the time), and the Satanists are present at her birth and know that she is "the one." It's safe to assume that the Vatican, and the Catholic Church in general, has spies/informants in the Satanists' society, and were able, in the intervening 20 years between Christine's birth and the events of the film, to track her down to New York and send Aquinas there (the fact that he was sent by the Vatican is established).
Question: How did the thugs who killed Jericho's family find out where he lived? Also, how were they able to break into his apartment without anybody doing anything such as trying to interfere and/or call the police?
Answer: They had followers everywhere. Cops, politicians, civil service workers and who knows who else. They could have easily found his apt. Just because they were thugs doesn't mean they don't know how to sneak in and out of places.
Chosen answer: Satan had to protect the host's body from harm by the Catholic priests who might try to stop him with holy "weapons." At the end, when Jericho impales himself on Michael's sword, Satan is cast out of Jericho's body because of the holy power of the sword.
Bishop73