In 8:28-39 Paul has spoken of the certainty of the believer’s hope. In 8:28-30 he has referred to God’s purpose as the ground of our certainty. But, according to the Old Testament, Israel had a special place within God’s purpose. The end of chapter 8 was therefore a natural point at which to introduce a discussion of the relation of Israel to the divine purpose. We may, in fact, go farther and say that at this point the need for such a discussion has become urgent, since the very reliability of God’s purpose as the ground of Christian hope is called into question by the exclusion of the majority of the Jews. If the truth is that God’s purpose with Israel has been frustrated, then what sort of basis for Christian hope is God’s purpose? And, if God’s love for Israel has ceased, what reliance can be placed on Paul’s conviction that nothing can separate us from God’s love in Christ (v. 38)?...
We shall misunderstand these chapters [9 to 11], if we fail to recognize their key word is ‘mercy.’ Paul is here concerned to show that the problem of Israel’s unbelief, which seems to call in question the very reliability of God Himself, is connected with the nature of God’s mercy as really mercy and as mercy not just for one people but for all peoples; to show that Israel’s disobedience, together with the divine judgment which it merits and procures, is surrounded on all sides by the divine mercy – and at the same time to bring home to the Christian community in Rome the fact that it is by God’s mercy alone that it lives.
It is only where the Church persists in refusing to learn this message, where it secretly – perhaps quite unconsciously! – believes that its own existence is based on human achievement, and so fails to understand God’s mercy itself, that it is unable to believe in God’s mercy for still unbelieving Israel, and so entertains the ugly and unscriptural notion that God has cast off His people Israel and simply replaced it by the Christian Church.
~ C.E.B. Cranfield, Romans: A Shorter Commentary