Paul speaks about the hardening of the people in the same sense in which the Book of Exodus speaks about the hardening of Pharaoh's heart before God...
Paul speaks about the hardening of the people in the same sense in which the Book of Exodus speaks about the hardening of Pharaoh's heart before God (Ex. 8:15, 8:32), and for him this is inseparable from reflections on God's providence and human freedom. Why, then, does a person become hardened? Humanly speaking, this is completely clear; but why God did not intervene is impossible to understand, just as it is impossible to understand why He did not intervene in the case of Pharaoh, whose heart was in His hand, as were the hearts of those who met the Savior during His earthly ministry.
Only this is obvious to the apostle: first, not everyone became hardened, and second, not forever. Here he recalls the story with the prophet Elijah, who was also sure that no one faithful to God remained except himself, until God Himself told him that he was not alone. As for the Jewish people, Paul again speaks of a remnant in roughly the same sense in which the pre-exilic prophets spoke of it. They too knew that not the whole people would accept the Messiah, but only its smaller part, which still had to be formed, and in the process of persecutions and catastrophes.
That is how it happened: the remnant that began to take shape even before the exile was persecuted by the authorities, and then, during the exile, the whole Jewish community of Babylon became the remnant. And this is not a figurative expression: in exile only one who kept the faith of the fathers could remain a Jew; the rest were doomed to rapid assimilation. In those times the remnant was distinguished by a religious mark, and the community that formed therefore became an ethnoconfessional community: in post-exilic times a Jew was anyone belonging to the Synagogue, regardless of whether he was born a Jew or accepted Judaism.
But when the Messiah came, it turned out that religiosity is not the main criterion. More than that, it became clear that sometimes precisely religiosity prevents a person from accepting the Messiah and entering the Kingdom. Paul, of course, knew this perfectly well, in particular from his own experience. Therefore he began to speak about the selection of another remnant, which would accept the Messiah and become the new people of God. But this new people does not replace the former one; it grows out of it just as post-exilic Jewry grew out of pre-exilic Jewry. Only now the main feature of the people becomes not its religiosity, but its readiness to live by grace.
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11:2 God hath not rejected that part of his people whom he foreknew - Speaking after the manner of men. For, in fact, knowing and foreknowing are the same thing with God, who knows or sees all things at once, from everlasting to everlasting.Know ye not - That in a parallel case, amidst a general apostasy, when Elijah thought the whole nation was fallen into idolatry, God "knew" there was "a remnant" of true worshippers.
11:3 Ki1 19:10.
11:4 To Baal - Nor to the golden calves.