In ancient Israel, when those from other nations dwelt among God’s people, they all had the same moral obligations; there was one law for everyone. We call it Torah, God’s instructions; it includes the Mosaic laws in Exodus, Leviticus and Deuteronomy. No difference was made between Jew and Gentile. (Nu 15:15-16)
I believe Torah, this same set of ancient laws, is timeless (Ps 119:160), defining holiness and sin for us today (1Jn 3:4); each one rooted in the eternal nature and character of God (1Pe 1:16), His testimony of metaphysical reality.
No one has ever been saved by keeping Torah (Ro 3:28); it’s purpose is to reveal God’s standard of holiness for His people. (1Ti 1:5) Since all of Torah is profitable for instruction in righteousness, thoroughly equipping us all to good works (2Ti 3:16-17), I believe all of Torah is generally applicable for every people group and culture for all time (Mt 5:19), for Jew and Gentile alike. (Mi 4:2)
When we willfully break any of these laws we’re guilty of breaking the Law as a whole, in its entirety (Ja 2:10-11), and we grieve and anger God. (He 10:26) God commands us to hide the words of His Law in our heart (De 6:6), and exhorts us to delight His Law and to meditate on it all the time. (Ps 119:97)
A general proof of this One Law concept is relatively straightforward:  Christ teaches Torah will be obligatory, relevant and applicable as long as Heaven and Earth remain. (Mt 5:17-18)  He also affirms that every single command in Torah has intrinsic moral significance as an expression and reflection of the one supreme moral standard: loving God with our whole heart, soul and mind, and our neighbors as ourselves. (Mt 22:36-40)  And finally, Christ commands His Jewish disciples to teach all nations to observe and obey all things whatsoever He has commanded them (Mt 28:19-20); God makes no Jew-Gentile distinctions in His commands.
Objection to One Law is nearly universal in Christianity, and generally passionate, opposing these basic principles by presuming that certain Torah commands have no timeless, intrinsic moral value, and then either  claiming Christ has abolished Torah altogether, or  arbitrarily classifying God’s laws into types (e.g. moral, ceremonial, civil, etc.), and presuming that certain kinds of laws (e.g. ceremonial and civil) apply only to Jews and not to Gentiles.
Motivation for opposing One Law is simple: the carnal mind is enmity against God and cannot be subject to Torah (Ro 8:7); our unregenerate nature cannot perceive Torah’s intrinsic moral value. But as we believe on Christ and partake in the New Covenant, God gives us new hearts (Eze 36:26) and begins to write Torah into our minds and hearts (He 8:10), putting His spirit within us and causing us to walk in His ways (Eze 36:27), moving in our inward man to delight in Torah. (Ro 7:22) As we grasp its immeasurable value (Ps 119:72), how it reveals the nature of God and Man (Ps 119:130), how all who delight in it are blessed (Ps 1:2-3), we’ll never strategize to limit its applicability or scope.
Many different scriptures are used to refute One Law, but I find that each one must be wrested from context to do so. (2Pe 3:16) So far, all opponents I have seen don’t consider that each text can easily be interpreted consistently with One Law, and ignore or superficially dismiss the key texts supporting One Law.
I think the definition of sin and holiness is one of the most important topics we can discuss; without holiness no one will see God. (He 12:14) Since my position here appears to be so unique, and since variations of the opposing viewpoint are so widely and passionately held, I invite and encourage any one to challenge me, and to engage in respectful, constructive dialogue for our mutual edification.