In many ways that ought not to be surprising. Paul sees followers of Jesus as the circumcision (Rom 2:29; Phil 3:3), the seed of Abraham and heirs according to the promise (Gal 3:29), the Israel of God (Gal 6:16), and as branches grafted into the olive tree of Israel (Rom 11:11-24). He repeatedly insists that there is no Jew nor Gentile, and that neither circumcision nor uncircumcision count for anything. In a sense, it would be odd if he did not regard Christians as ex-Gentiles, especially since he urges his converts not to live like the ἔθνη (e.g. Eph 4:17). Yet it may still jar with us, all the same. (It did with me, the first time I realised that’s what Paul is actually saying in 1 Cor 12:2). And if it does, I suspect it simply reveals the radicalism of Paul’s reframing of national and religious identity in light of Christ, and the extent to which we find it difficult to grasp, for all we affirm that the boundary line between Jew and Gentile has been destroyed.
This, intriguingly, amounts to supercessionism in reverse. It is not that Israel has been superseded by the Gentiles, leaving the Jewish heritage of the people of God behind (by no means!) It is, in contrast, that our Gentile heritage has been superseded up by our new Israelite one, as we are united both with Christ and with the whole Israel of God, past and future. When we used to be Gentiles, we used to worship idols. But now we are Gentiles no longer, we confess that Jesus is Lord by the Holy Spirit (1 Cor 12:3).