THE ONE-LAW PROPOSITION
All covenantal members of Israel are bound by "One Law" (Exodus 12:48,49; Lev. 24:22; Numbers 9:14; Numbers 15:15-16). This is binding precedent. Since the Apostolic Writings confirm that non-Jewish Believers are covenantal members of Israel, precedent dictates that non-Jewish are bound to this One-Law. Furthermore, the Apostolic Writings, in agreement with the Prophets, hold that non-Jews are covenantal members of Israel compelled by the Ruach haKodesh to keep the "One-Law".
OVERVIEW OF EVIDENCE FOR THE ONE-LAW PROPOSITION
(1) Yeshua's community is called the "Ekklesia" in the Greek source text of Matthew 16:18-19. Although Christianity has mistakenly translated this term as "Church", Ekklesia, in its Hebraic context, refers specifically to Israel (Acts 7:38, also see LXX of Dt 9:10; 10:4; 18:16; 5:22). This means that all Believers, whether Jewish or non-Jewish, belong to Israel.
(2) Ezekiel 36 explains that the New Covenant involves a washing with water, a cleansing by the Spirit, a compulsion to keep Torah (both chukim and mishpatim), and offers permanent forgiveness of transgressions. In Acts 2, on the day of Shavuot, the day commemorating the giving of the Torah at Sinai (known in the Torah as "the day of the Kahal"), Peter explained that the gift of the New Covenant was being offered not only to the Jews who were present but also to the non-Jews who were "far off" (Acts 2:39 and Ephesians 2:13). In Acts 11:15, Peter recalls his amazement that the Ruach had been given to non-Jews just as it had been given to Jewish Believers on Shavuot/Pentecost ("...the Holy Spirit fell on them as it had fallen on us at the beginning"). That the Apostles Peter and Paul considered this to be proof that the non-Jews were covenantal members of Israel is further evidenced by passages such as Ephesians 2:12 and 1 Peter 2:9-10.
NOTE: The term in Ephesians 2:12 mistranslated as "commonwealth" is "politeia" which is a term that Luke himself uses in Acts 22:28 to refer to citizenship. Shaye Cohen explains further, "The Greek word politeia means in the first instance 'citizenship,' the quality of being a citizen," (Cohen, The Beginnings of Jewishness).
(3) In Acts 15, there is a debate over covenantal initiation: was it circumcision or faith that was initiatory? Peter argued that it was faith (Acts 15:9) and grace (Acts 15:11). He also argued that these Gentiles had become "a People for God's Name", an expression that refers specifically to Israel. It is evident in the Tanak that only Israel is called by G-d's Name (see Deut. 14:2 and Isaiah 63:19) and Peter himself makes this explicit in 1 Peter 2:9-10 by applying exclusive titles of Israel to Gentiles.
Also important to note is that James' Speech which appears to misquote the LXX version of Amos 9:11-12 is in fact a reference to many Prophetic passages (note James' plural use of "prophets" in Acts 15:15), each alteration to the LXX text forming a linguistic connector to many related Prophetic passages (see my previous blog post here). Here is one of these many references which communicates that the non-Jews are grafted into Israel as covenantal members:
For more on the significance of the Fourfold Decree, see here and here.