The animated opening was more of a compromise solution because the studios had no interest in going over budget for a live-action version of the century-long battle of humans against vampires, so the scene was done by animation which was much cheaper to realize.
This is Paul Bettany's fourth time playing a religious character. He played a monk in The Da Vinci Code (2006), the Archangel Michael in Legion (2010),a priest in The Reckoning, and a priest in this movie.
The trivia item below may give away important plot points.
Director Scott Stewart originally wanted Brad Dourif's character to appear as a human/vampire-hybrid in the second half of the movie. Unfortunately, Dourif was only available for two days of shooting, so Stewart 'had to kill him off'