First, Galatians 3:29 does not say we are made partners in the New Covenant. It says that we are made heirs to the Covenant of PROMISE made 430 years before that of Law. This was not made with Israel, but Abraham and his SEED. There are three "seeds" promised to Abraham:
- Christ (Gal.3:16)
- Seed as the sand of the sea-shore - earthly - Israel
- Seed as the stars of heaven - heavenly - the Church (Heb.3:1)
It pointedly says that we are seed NOT BY BEING Israelites, but by being Christ's. This refers to John 12:24 were we are the many seeds that came out of Christ by the New Birth. But by virtue of Christ being the Chief Seed of Abraham, we who are IN Him become eligible to inherit the world (Rom.4:13). Abraham was never ever an Israelite - but a Gentile - a Syrian.
Second, Ephesians 2 does not, at any time say that we become Israelites. It pointedly annuls this notion by the use of the word "COMMONWEALTH". A "Commonwealth" is not one nation. It is the enjoyment of common good for multiple nations under a sovereign or government. A New Zealander does not become a Brit because he is in the Commonwealth. But he is eligible for the privileges that come from Her Majesty. The "Common" - "wealth" of British Colonies does not make a Nigerian a Scot.
Third, there is not a single case of a believer becoming an Israelite. It would be impossible since the believer is a "New Creature" and old things have passed away (2nd Cor.5:17). This includes ethnicity (Gal.3:28; Col.3:11). Israel and the Church remain separate throughout the New Testament into all eternity. The Church are the walls of New Jerusalem while Israel are the Gates.