Do you have beliefs? Of course you do! You are a real person with your own opinions on religion, politics and morals; so why shouldn’t your characters be the same? After all, you want them to feel real to the readers, you want your audience to connect with them. Give your characters beliefs and opinions and then ask yourself: would they stand up for those beliefs in public? Are they covert about them? Are your characters fanatics? What happens when someone questions their beliefs? How all that would affect your story?
Even if you don’t include all the answers in your book, you’ll have a better understanding of your own characters by answering these questions.
Check out TTWT #16 for more on how your character’s beliefs fit into your writing.