Paul seems to have been aware of a wrong attitude among some Gentile converts regarding the Jews. Those Gentiles had assumed that God was finished with Israel. So Paul used the illustration of an olive tree to depict what had happened and what would yet happen. The point that Paul made is that since the Gentiles, although unnatural branches, could be grafted in to the olive tree, even so God could graft in the natural branches in the future.
In this set of verses we have one of the New Testament mysteries. A mystery in this sense is not something obscure. Instead it is something that has been revealed by God. The mystery is that a partial hardening has come upon Israel until the fullness of the Gentiles comes in. There are some difficult statements here to understand.
One is, whom does Paul mean by Israel? His usage of Israel in verse 25 is different from what he means by Israel in verse 26. In verse 25, Israel includes Jews who have been judged by God whereas in verse 26 Israel is composed only of saved people. Some people think that Israel in verse 26 is used in a spiritual sense and includes as well the Gentiles mentioned in verse 25. But I think that suggestion does not make sense in the passage because all the other references refer to literal Israel. So I would say that what Paul means by Israel in verse 26 is all converted Israelites from throughout history, including those Jews who will be converted in the future.
Second, what does Paul mean by the fullness of the Gentiles (v. 25)? He says that this fullness will be completed during the same period when literal Israel is partially hardened. Some say that this means that the conversion of Israel as a race in the future must therefore be the final saving act of God before Jesus returns, and they would say that ‘fullness’ here means all converted Gentiles. They would claim support for this interpretation from verse 15, which they read as indicating the conversion of Israel as a race is followed by the resurrection.
The problem with this interpretation is that it contradicts what Paul says in verse 12 where he writes that the ingathering of Israel will bring about greater blessings for Gentiles than the cutting off of Israel did. So I would suggest that ‘fullness of the Gentiles’ in verse 25 means that a large number of Gentiles will be converted before Israel is restored. Perhaps some readers would have assumed that nothing much would happen until Israel would be changed. Paul writes that great things will happen when Israel is restored, and he also says that great things will happen before they are restored.
Third, how will Israel be restored? Paul answers this question in verses 26 and 27 by citing some Old Testament statements. Using them, Paul tells his Roman readers that a wonderful day is coming when Jesus from heaven will bring the Jewish people to repentance and enable them to become a holy people. And that day will be a great time of gospel blessing for the nations as well.
Fourth, God shows grace to the disobedient (vv. 28-32). Paul reminds his Gentile readers that they should regard Israel from two perspectives. One is that with regard to the gospel, Israel is an enemy of God in order that blessings would come to the Gentiles. The other is that God has elected Israel to have a special role in his plan of salvation. He made unchangeable promises about them to their forefathers concerning their seed being a source of blessing for the world. They are a cause of blessing in their disobedience because salvation has come to the Gentiles and they will be a source of blessing in the future for Gentiles when as a race they return to God and embrace their Messiah. God works to a plan and central to that plan is that he will show mercy to undeserving, disobedient Gentiles and to undeserving, disobedient Jews. And nothing, says Paul, can prevent God doing this.