For a general introduction to call numbers and ranges see our Library of Congress Classification Guide.
These are some Library of Congress call number ranges relevant to theology:
B is the call number range for philosophy, psychology, & religion. Sample subclasses:
Remember to not only look for titles about your specific topic, but for titles that appear to be about a broader topic that your topic fits within.
Look inside individual books to get a sense of their content. The best places to start are:
This will be at the front of the book and will tell you chapter titles. Some books are better than others at having descriptive chapter titles that signal their contents.
When a book has an index (or sometimes multiple indexes) will be located at the back of the book. An index is a list of names, concepts, or keywords and the pages of the book where content on that topic can be found. Constructing a high quality index is labor-intensive, so not all books have them or have equally extensive indexes. A good index, however, is incredibly helpful for finding information.
Dugan Library has a special collection of works related to St. John Henry Newman located on the first floor of the library. Don't forget to browse there in addition to checking the stacks on the second floor.