What does Romans 11:23 mean?

Paul has already implied that one day Israel will return to her special relationship with God as her people come to faith in Christ. Paul seems confident that is Israel's future. Now, for the first time, he adds that being pruned from God's olive tree, as an unproductive branch is broken off, is not necessarily the end of the story for these Israelites. The reason they were taken off the main trunk was a refusal to believe in Christ as the Messiah and an insistence on proving their righteousness to God by following the law.

Paul now writes, though, that these discarded branches can be grafted back onto the tree if they don't continue to reject Christ in their unbelief. Paul is clear: God has the power to do this. He can grant His people Israel their rightful place on the olive tree when and if they trust in Christ. This ability to be grafted back in, after being "cut off" is further evidence that Paul's discussion here is not about eternal salvation, but about fellowship with God.