Ancient historians used to treat state religion, the temples, priesthoods and public festivals that a city or empire maintained on its own behalf, as the whole of ancient religious life, with whatever happened inside a private house dismissed as superstition too small to study seriously. The household and family religion movement in classical and Near Eastern studies has reversed that judgment. Public religion was never universal; participation in a city's official cult could depend on citizenship, class or gender, and vast stretches of ordinary religious practice, keeping the hearth, feeding the household gods, marking a birth or a death, happened entirely outside any temple's reach.
The Roman household is the clearest surviving case. Alongside the great state cult of Vesta, tended by the Vestal Virgins on behalf of the whole city, every private house kept its own household hearth and its own Lares and Penates at a lararium, a shrine maintained by the family itself with no priest required. The two systems ran side by side rather than in competition, public religion answering for the city, household religion answering for the family, and the same doubling recurs elsewhere: Hindu temple worship coexists with daily puja at a home shrine, Japanese shrine Shinto coexists with the household kamidana, and Chinese state and temple ritual coexists with the kitchen god and the ancestral tablet kept at home.
Reading household religion as its own category, rather than as leftover folklore beneath a tradition's real theology, is what lets a single household altar be read as evidence of a functioning religious system rather than as evidence of nothing worth recording.