What does Romans 11:29 mean?

Paul has written in previous verses that God's people Israel will return to God. This will be through faith in Christ, at some point in the future. Through election—God's sovereign choosing beforehand—this is the purpose for which God has destined the nation of Israel.

Why has God done this? The prior verse said that it is because of their forefathers. Those are the patriarchs of Israel: Abraham, Isaac, and Jacob. God gave to these three men great and unconditional promises that He would bless them and make of them a mighty nation. Since these promises were given by God, they carried the weight of certainty. The promises were God's gifts. God called the patriarchs to follow Him and He made an everlasting covenant with them that included their descendants.

Paul writes that these gifts and calling are irrevocable. Though God later made conditional covenants to bless Israel if they would keep the law, His promises to the patriarchs were mostly one-sided promises based only on His own goodness. Those promises will not be revoked no matter how deeply Israel sins in rejecting faith in Christ as the Messiah. He will always call Israel back into relationship with Himself, even bringing the people of the nation to faith in Christ in due time.