The title should give you a clue, but it loses a lot in translation to film. The whole point of Angels & Demons was to explore the duplicity of science & religion, piety vs humanity, discovery & destruction, good & evil coexisting simultaneously. If Robert Langdon were not a pivotal character already, you wouldn't need him to tell you that this story is a symbologist’s dream. However, the complicated levels of relationships that are not present in the movie weaken the story line and undermine the very foundation of the events that make the title so revealing. Another level is lost without more reference to the duplicitous architectural elements of the Roman landmarks that lend so much weight to the backstory.
Literally, this movie is right out there! Leave your reality-based expectations at home and just take in all the action, the special effects, the fun of seeing the changes in Harrison Ford and Karen Allen, and try to appreciate the imagination of George Lucas and David Koepp. It's fast paced, has its humerous moments and goes way beyond good versus evil. Kind of like "The Mummy" meets "The Dark Crystal". Plenty of homage to prior Indy adventures and characters, its a fun trip down memory lane with some new life in the form of Shia LaBeouf. Don't try to figure it out the first time around, just enjoy it.