1. Scripture's own definition: "they are not all Israel, which are of Israel"

From Sovereign Grace and the Scope of Election: A Comparative Examination of Attainment Theology and Grace Theology, with Particular Attention to the Question of Whether God's Elect Are Found Among All Nations and Religions

Paul states the principle in exactly the words needed: "For they are not all Israel, which are of Israel: neither, because they are the seed of Abraham, are they all children... That is, They which are the children of the flesh, these are not the children of God: but the children of the promise are counted for the seed" (Romans 9:6-8). Fleshly descent never constituted the true Israel, not in 1948, not in Paul's day, not in Moses' day. Within national Israel there was always a smaller, chosen Israel: "Even so then at this present time also there is a remnant according to the election of grace... Israel hath not obtained that which he seeketh for; but the election hath obtained it, and the rest were blinded" (Romans 11:5, 7). Notice how naturally this locks into the thesis of this whole document: "Israel," in its saving sense, has always meant the election of grace, never a bloodline, never a border, never a flag.