Xenia

Xenia governed the relationship between host and guest in the ancient Greek world, and it was considered a matter of religious obligation rather than mere social courtesy. A host was expected to offer food, shelter, and protection to a guest without demanding their identity first. A guest, in return, was expected to be grateful, undemanding, and respectful, and ideally to reciprocate the hospitality if roles were ever reversed.

The custom carried divine weight because Zeus himself, in his aspect (or epithet) of Zeus Xenios, was believed to watch over strangers and travellers. Meaning that any guest might be a god in disguise, and any breach of xenia was an offence against Zeus. The myths are full of cautionary examples:

The swineherd Eumaios sheltering a disguised Odysseus is xenia honoured.

Paris abusing Menelaos's hospitality by absconding with Helen is xenia violently broken, and is framed by Homer as the roof cause of the Trojan war. 

For modern practitioners, xenia is often interpreted as a living ethic. Generosity towards guests, courtesy towards strangers, and a broader value of reciprocity and hospitality, rather than a literal, ritual, observance.

See also: Zeus Xenos