"The stipulations of God's moral law, whether known through Mosaic (written) ordinances or by general (unwritten) revelation, carry a universal and "natural" obligation that is appropriate to the Creator-creature relationship, apart from any question of redemption. Their validity is by no means restricted to the Jews in a particular time period...At the beginning of the book of Deuteronomy, when Moses exhorted the Israelites to observe God's commandments, he clearly taught that the laws divinely revealed to Israel were meant by the Lawgiver as a model to be emulated by all the surrounding Gentile nations, Deut. 4:5-8. All the nations, not just the Israelites, should follow the manifestly righteous requirements of God's law. In this respect, the justice of God's law made Israel to be "a light to the nations" (Isa. 51:4).
Unlike many modern Christian writers on ethics, God did not have a double standard of morality, one for Israel and one for the Gentiles ( cf.* Lev. 24:22). Accordingly, God made it clear that the reason why the heathen tribes were ejected from the promised land was precisely because they had violated the provisions of His holy law (Lev. 18:24-27). This fact presupposes that the Gentiles were antecedently obligated to obey those provisions. Accordingly, the psalmist condemned "all the wicked of the earth" for departing from God's statutes (Ps. 119:118-119). Similarly, the book of Proverbs, intended as international wisdom literature, directs all nations to obey the laws of God: "Righteousness exalts a nation, but sin is a disgrace to any people" (Prov. 14:34, see also Isa. 24:5)."
--Greg Bahnsen, Five Views on Law and Gospel (Zondervan, 1993, pp.110-111)
* cf. is the abbreviation for the Latin "confer," meaning "compare" or "consult."