Israel was chosen by grace and not by any righteousness of their own.
(Rom 9:11)
Same with Gentiles, as we see NT writers tell of Gentiles believers being blessed instead of ethnic Israelites (as with Naaman).
So God's "elect" were always believers of any nation, and none of them could boast about themselves over the others.
(Rom 11:18)
Paul expresses this very concept when he uses the Hosea passage of "not my people" becoming the people of God once again and applies it to Gentile believers
(Rom 9:25-26) as much as Hosea applied it to Israel
(Hos 10:23).
And no surprise since Paul was keen on the words of the OT and encouraged all to search those OT scriptures to see if what he told them was truth or not.
(Act:17:11)
Romans 10
(19) But I say, Did not Israel know? First Moses saith, I will provoke you [Israel] to jealousy by them that are no people, and by a foolish nation I will anger you.
Quoted from the song of Moses in the OT:
Deuteronomy 32
(21) They [Israel] have moved me to jealousy with that which is not God; they have provoked me to anger with their vanities: and I will move them [Israel] to jealousy with those which are not a people; I will provoke them to anger with a foolish nation.
Saved by grace was not some newfound mystery just for "Gentile church believers", it was for believers of the whole world all grafted into the same tree together as the people of God whose root was God/Christ.
(Rom 11:16-18)