Marriage was a fundamental institution of society in ancient Rome and was used by Romans primarily as a tool for interfamilial alliances. The institution of Roman marriage was a practice of marital monogamy: Roman citizens could have only one spouse at a time in marriage but were allowed to divorce and remarry. This form of prescriptively monogamous marriage that co-existed with male resource polygyny in Greco-Roman civilization may have arisen from the relative egalitarianism of democratic and republican city-states. Early Christianity embraced this ideal of monogamous marriage by adding its own teaching of sexual monogamy, and propagating it worldwide to become an essential element in many later Western cultures.