Consistently in Isaiah, “Israel” appears as a heterogenous group. God in Septuagint Isaiah does not give promises of salvation as a blanket statement to the nation of Israel as a whole. Rather, he gives his promises to his people. Sometimes he calls these people “Israel,” or “Zion.” But chapters 56-57 indicate in plain speech that God does not welcome into his assembly everyone found to be of Israelite descent. …
The “Barren”–Faithful Israel: Devotional 2.57
Paul’s thought, which he apparently garnered at least in part from Isaiah, is that the people of national Israel of the Old Testament will not all be saved. Only a remnant of ethnic Israelites, those who try in the long run to follow God, rather than rebel against him, receive God’s promise of salvation. The idea is that a necessary aspect of salvation includes belief in God (which means a faithful following of his way). God excludes the persistently disobedient from his promises of blessing, comfort, and salvation. …