One theme that is present in this Dan Brown book is the blending of science and religion. Throughout the book, readers see both sides of the debate. The supporters of science, knowledge, and freethinking would be the Illuminati, and the religion supporters are obviously the Catholic church. Finally, the book does show that it is possible that science and religion don't have to be in opposition to each other. Leonardo Vetra and his daughter show readers that the two methods of thinking can find harmony and operate together.
A second theme is that of religious corruption. While Langdon does interact with plenty of devout and completely moral religious characters in the book, the plot is propelled forward by the work of corrupted religious figures such as the Pope himself. Additionally, the Illuminati have infiltrated the papacy, and Cardinals are being murdered in order to manipulate the future papacy. All of this works to give readers a general sense of distrust toward a religious institution that historically is supposed to represent trustworthiness and moral uprightness.