Books are shelved in general subject categories using the Library of Congress classification system. You may want to supplement your use of the catalog with browsing shelf areas for your topic. Below is a brief listing of some of the subject locations in the field of Religion.
You may also find sources in the field of History, specifically:
Searching the catalog (above) will lead you to books at Gustavus. It will also connect you with books at many other libraries in Minnesota (select All Libraries to search once you're in the catalog). Explore these links to find books at other libraries & online.