This habit of swearing by things, especially gods, is one that became so habitual for people that has ceased to have much meaning, to get real power out of an oath anymore, you generally have to swear on a loved one’s grave, and perhaps elaborate how cherished such a person was, before your oath would gain any particular credibility. But this – then – this swearing by the gods, not even any specific ones, was enough to give your oath some extra power back in the day.
The Cymbeline Project 