God’s promise to Israel — that covenant promise that begins at Genesis 12 — is, according to Paul, “irrevocable” (Rom. 11:29). What is now will not be the way things always will be. As Gentiles have now been moved into that covenant blessing, so “Israel” will be eventually.
This means the Gentiles (Paul’s audience) cannot become arrogant about their status.
The “gifts” (cf. 5:15-16; 6:23) are irrevocable; the blessings of redemption to Israel have been promised by God and his word is true.
And Paul shows the way God works: “For God has imprisoned all in disobedience [Israel now; Gentiles prior to the Messiah] so that he may be merciful to all” (11:32).