Relation of My Imprisonment

By SF I mean speculative fiction. This is an alternate history, where an entire religion has sprung up around the worship of the dead. Banks has invented a Bible and theology that's highly interesting, and this book is worth reading for just his creation of an interesting world.The book itself is a form of Puritan works, sent by the jailed practitioners to the congregation. Banks manages to capture the austere yet somehow elaborate Puritan method of writing, while at the same time keeping the book interesting for modern readers.The only point I'm really not clear on is...well...his point. This is obviously intended as some sort of allegory; why go to the lengths he did just for his own amusement? But I'm not sure WHAT this allegory is about? Email any suggestions, I'm genuinely curious as to theories.