Reference books like dictionaries and encyclopedias are great ways to get background information on your topic.
Reference books like commentaries and concordances are especially useful for religious studies. Commentaries give detailed descriptions of verses in the Bible. Concordances are verbal indexes to the Bible--you can see each instance where certain words appear.
You will need to cite information you learn from reference works, just as you would from books and articles. Citation information is usually at the bottom of the screen if you are using an online reference source.