Rice brings a number of strong religious themes to this novel as a result of her research into ancient Hebrew and Babylonian cultures as well as modern Hasidic society.
The most obvious theme is that of ritualistic sacrifice. The Servant of the Bones is created through a tremendous, if somewhat unwilling, act of self-sacrifice so that his people could be freed, and that spirit of service to God may have prevented Azriel from becoming the evil spirit his creators had intended. Rachel Belkin is touted as a sacrifice by the Temple of the Mind, but that assessment may not have been far from the truth, since her death seems to have triggered Azriel's reemergence from the Bones.
Finally, Gregory Belkin sacrifices his own brother for the furtherance of the Temple of the Mind, and Azriel sacrifices a chance to enter Heaven in order to stop Belkin.
Beneath the plot...