God has not rejected Israel—Paul himself, an Israelite, is evidence of that! And now, even though it appears that all Israel is apostate, God has reserved many for himself. This ‘reserve’ is completely in line with God’s purposes in election, because it means that salvation is not by their work or heritage, but rather by God’s grace alone. This remnant exists even now, while those who remain are hardened in judgment.
What does this mean, then, if we are ‘in’ and many are ‘out’? It means that a new dynamic has emerged. And as David proclaimed his faithfulness and condemned those who persecuted him, now, in relation to the faithful, those who reject Christ have become the enemies of Israel. The elect remnant has obtained the true hope of Israel, while those who remain are hardened in judgment.
The elect, then, while provoking persecution by the unbelieving, also provoke jealousy. And Paul says that the very purpose of our salvation is to enflame with jealously those who are still in rebellion against God; having obtained the hope of Israel, we now make national Israel jealous for her hope.
Paul closes with a question: if by rejecting God national Israel generates this effect, what would be the effect of their acceptance and return to God? When Jonah ran from God, the sailors on the boat ended up praying to God. Even disobedience produces fruit. But, we can speculate, what kind of fruit would be produced by obedience?