In his recently published book, Paul and the Vocation of Israel (BZNW 205. Berlin / Boston: Walter de Gruyter, 2014), my good friend and colleague, Lionel Windsor, discusses how Paul viewed his own vocation as Apostle to the Gentiles within the larger picture of Israel’s vocation as the classic people of God. Lionel has recently blogged about one of Paul’s often misunderstood phrases: the ‘Israel of God’ in Galatians 6.16. You can read Lionel’s short summary here, and even download an electronic copy of chapter 3 from his book.
I recommend Lionel’s work highly.