For reference, see Ed’s post here, summarizing ANE (Ancient Near East) feudalist social structure.
Noragami is about gods and their work in Japan as they battle phantoms that plague the country’s citizens. The gods are essentially humans in form, with obvious special powers, mostly invisible but can appear to anyone if they decide to. All the gods utilize Regalia (shinki), who are humans who have passed from the Near Shore—the land of the living—under unfortunate circumstances. The wandering human spirits are adopted in the god’s service at his pleasure, thought adoption is too weak a word to describe the relationship. Regalia are essentially covenanted into the god’s service, but are under little obligation to stay. Regalia normally have nowhere else to go except into another god’s service, so “god-hopping” is unusual and rather frowned upon, as serving more than one master, especially at the same time, is distastful.
Regalia are called into service by the god granting the human spirit some of his life force and giving it two names: a new human name and a vessel name, as means of establishing the contract. The spirit then becomes human in form, like the god, and just as invisible. When the god calls the vessel name, the Regalia transforms into some object to defeat the phantom. Usually it’s a weapon, but it can also be an assisting tool, or even an animal or mode of transport.
Yato, a god of calamity, recruits a new Regalia:
It’s worth mentioning that the connection between god and Regalia goes much deeper than slave and master, or even parent and child. The two are linked much more closely, so that what the Regalia does and feels actually affects the god physically. The result is that Regalias risk hurting their master if they make immoral decisions, or get upset or angry. So the Regalia has a duty not only to obey their master, but keep their own sense of well-being. In turn, the god offers the Regalias protection, guidance, and a sense of belonging to a family—all things they didn’t have when they were an isolated spirit. Many of the more successful gods have physical shrines dedicated to their existence, but that comes as a reward for dedicating their power to help humanity. The core of their existence as gods comes from their relationship with their Regalia and their combined efforts to answer the prayers of Japanese citizenry.
In Noragami and countless other series like it, you’ll notice a strong presumption of the supernatural interceding in everyday Japanese life. “First contact” with beings or phenomena are met with merely mild surprise, as though it were not precisely predictable but still inevitable. Contrast this with a more Western view of the supernatural in film and television, in which human characters presume malice on the supernatural world; ghost stories intersect very strongly with horror.