It's usually best to start with a keyword search, but subject headings applied to relevant books can help to refine your results. Open the item record of a book that interests you. Scroll down to the clickable subject headings, and run another search to explore similar books.
You can also browse the headings associated with a particular topic as a way of getting started with your research.
B Philosophy. Psychology. Religion
B - BJ Philosophy. Psychology
BL, BM, BP, BQ Religion: Religions, Hinduism, Judaism, Islam, Buddhism
BR - BV Religion: Christianity, Bible
BX Religion: Christian Denominations