What is the theme in The Gate of Angels by Penelope Fitzgerald?

Religion is a major theme in this book. Flowerdew tells Fred that the "earlier" view, that Nature or an invisible god has created everything with a purpose in mind, has, after all, been widely held for centuries. Most regard religion as a matter of specific beliefs: The Benedictine nun accused of trying to poison the disputed pope was burned at the stake, and Fred's family history contained many heated religious feuds. Nevertheless, he thinks it "simple decency" to tell his father that he no longer takes anything on faith.