What is the theme in Catherine, Called Birdy by Karen Cushman?

Religion plays an extremely important role in Catherine, Called Birdy, as it did in English society at this time. This is most immediately apparent in the book of Saints that Catherine has - each day belongs to a Saint, and she delights in telling their stories. At this time, people who could read and write mostly did so about religious stories. The importance of Saint's days is also underscored by the manor's celebration of them - they appear to hold as much significance, if not more, than the character's actual birthdays.