It describes the age long competition of science versus religion. Scientists like Leonardo Da Vinci, while attempting to appease the church, often delved into areas forbidden by the Vatican etc. Many scientists of that era walked a thin line. Subsequently they formed a secret organization whose purpose was to advocate science despite the Catholic Church. Some members were actually within the Catholic Heirarchy, though it was kept secret. In Angels and Demons, the competition between the 2 comes to a head... the movie has a significant amount of factual basis..