Religion and death
Because Giovanni Bruno survived death, they worship him as a kind of deity in his new cult—the Brunists. He didn't ask for them to form a cult, of course, so the accidence of it shows something important about human nature. People are informed by near-death experiences. And in relation to the concept of death, humans build religions. The theme is a portrait for where religions come from—they come from humans trying to creatively understand the problem of death.
The occult and spiritual
There is a strong picture of witchcraft-kinds of magic, like Eleanor's channeling of spirits, especially the formidable voice in her head, Domiron. As his name suggests, Domiron rules her with iron dominion. He is a burden to her, but this hypothetical (perhaps imaginary) relationship allows Eleanor to do what she feels instinctually obligated to accomplish, because she is an occult spiritualist, and also, she seems a little beyond sanity, to put it nicely.
Cults
The novel is a depiction of cult. Cults are formed when people form a community around the charisma of a person. In this case, Bruno's charisma is his experience of comatose states and near-death experience. These make him a valuable point of view on the important issue of human death and the meaning of human experience. Of course, the beautiful part of this plot is that Bruno didn't form the cult intentionally. It rose up naturally around him, as a suggestion that cult-formation is part of human nature.
