Thanks for the information about your grade. There are many themes in this book that you can develop. So, I will just name a few. First, one of the most obvious themes of the book is the idea of conspiracy. So, anything thing that deals with a conspiracy theory would work. This is a great topic, because it is also very fun to think about.
Second, there is the theme of symbols and how symbols function. This is also interesting, because there are so many symbols in our world today. To interpret these symbols might be fun.
Third, you can write on the Catholic church, since the church plays a prominent role in the book. Finally, you can talk about the theme of secrecy. As you know there are so many secrets in the book.