In the book of Acts, we find that a time came when Paul turned to the Gentiles with the gospel after following the command to take first the gospel to Israel:
(Act 13:44 KJV) And the next sabbath day came almost the whole city together to hear the word of God.
Paul was persecuted by the unbelieving Jews, but he remained steadfast in proclaiming the gospel. He told the unbelieving part of the nation that he had delivered to them the way of salvation, but they rejected it and thus judged themselves unworthy of everlasting life.
Paul didn't judge them. Paul says it is they who judged themselves as unworthy of eternal life because they rejected the gospel of Christ. But, that didn't stop Paul from travailing over them and loving them. In the opening verses of chapter 9 of Romans, Paul bares his heart concerning his "kinsmen according to the flesh," meaning the unbelieving Israelites:
(Romans 9:1 KJV) I say the truth in Christ, I lie not, my conscience also bearing me witness in the Holy Ghost,
Paul knows his being cut off from Christ would not save them, yet, with his knowledge of Christ, he knows there is nothing he can do for them because they must individually believe on Christ to be saved. He states that the Israelites had heard everything they needed to hear in order to be saved - the adoption, the covenants, the promises - and not only that, they were the ones through whom Christ came into the world, yet not all believed and these who would not believe were cut off from the Christ. Paul goes on to say, that, lest anyone should blame the word of God for failing, God's word didn't fail. They just didn't believe and in rejecting the good news pronounced their own judgment. He had called them kinsmen after the flesh, and he travailed over them, yet he now brings in another group, the seed of Abraham as the children of promise:
(Romans 9:6 KJV) Not as though the word of God hath taken none effect. For they are not all Israel, which are of Israel:
Paul distinguishes between Israel after the flesh and Israel after the promise. Israel after the flesh were those who sought righteousness under the law, while the children of promise were those who acquired a righteousness that came through faith in Christ. He refers to both groups as one, but only in reference to being born after the flesh, calling them all children of Abraham (or natural Israelites): "For they are not all Israel, which are of Israel." "Which are of Israel" refers to all the Jews, both the Jewsish believer and the Jewish unbeliever, but then Paul paints a picture of the believing Jews within natural Israel who are also called Israel, those born after the promise given to Abraham. This second group are those whom God chose to enter into a special relationship in order to bring His plan of salvation to the world. Paul said this election is by God's mercy and a sovereign act in His plan:
(Romans 9:14 KJV) What shall we say then? Is there unrighteousness with God? God forbid.
God's salvation is to show His mercy, not only to the Jew, but to the Gentile as well. For millennia, the Gentiles were not God's people, while He clearly states that the Israelites were His people. But now, since the furtherance of God's plan of salvation in Christ was rejected, Paul says even the Gentiles can come to a place where God says they, too, are His people:
(Romans 9:24 KJV) Even us, whom he hath called, not of the Jews only, but also of the Gentiles?
Paul, of course, is speaking of those Gentiles who would come to believe on Christ, but then he goes on to add them to the remnant of natural Israel:
(Romans 9:27 KJV) Esaias also crieth concerning Israel, Though the number of the children of Israel be as the sand of the sea, a remnant shall be saved:
In Paul's time, some Jews and some Gentiles did not believe and therefore "stumbled at that stumblingstone." But, the remnant of Israel who came to righteousness through faith in Christ and the Gentiles who came the same way were one body in Christ. In the verses from Romans 9, Paul, therefore, set up four groups:
The Jews and Gentiles who would not believe on Christ are the unbelievers. The Jews and Gentiles who would believe on Christ are the believers. So, all that Paul said can be summed up in these two groups.
This must have been a very heartbreaking chapter for Paul to write. You can almost feel his anxiety over Israel and the pleadings of his heart, and this sorrow and travailing went on in the next two chapters as well.