What is the theme in The Hollow Hills by Mary Stewart?

New Religion vs Old Religion is central to the story. Around the 5th century, England began to have a major upheaval of its culture. This was caused by the spread of Christianity around the world. The nobles and royals began to follow Christianity (some by faith, others by selfishness). However, the Roman gods and goddesses were still prevalent in many of the smaller regions and towns in England. The commoners still worshipped shrines and gave their tithes and blessings to whichever deity they believed watched over their town