Dan Brown's Angels & Demons depicts the struggle between religion and science. For hundreds of years, the church has been at odds with modern science, as evidenced by Robert Langdon's referencing the history of persecution that the church embarked on to silence the voices of great scientists, Galileo.
In particular, the book is about a power struggle, within the church, which is depicted by the rogue Camerlengo's behavior. He wants to preserve tradition in an absolute way, while other clergy want to be more inclusive of modern science and thought.
The book depicts a race against time to save the cardinals who are marked for execution using symbolism that is known to be part of an ancient society the Illuminati. What is striking about the plot twist is that a member of the clergy uses a supposed arch enemy of the church to execute those cardinals that he does not want as Pope.
In conclusion, I would sum up by saying that the book is a fictional depiction of a power struggle that occurs within the Roman Catholic church regarding the opposing forces of tradition and a more modern inclusive approach.
This struggle is actually real in one sense, the church constantly discusses old traditions, such as clergy not being able to marry, no women clergy, but they go round and round, and things never change. So Dan Brown imagined that that one member of the high ranking Vatican clergy got so deeply involved in a plot to make sure that discussions of changing any traditions in the church never went any further.
Protecting tradition, in the name of God, using murder, deception and a willingness to die for the cause.