In antiquity and beyond, oaths between parties were usually sworn by a divinity both believe in. However, oaths could also be sworn between members of different religious communities. This poses several problems: by what divinity will the parties swear? How can one party trust another’s oath? By what ritual will the oath be sworn? What does such an oath mean for the parties’ religious identities and power relations? This article will survey Roman, Jewish and Christian texts (1–3 cent. CE), in search of evidence for oaths between private parties to outline possible responses to these questions.