In this passage, Paul explains how the nation of Israel did not receive imputed righteousness, but Gentiles did. First, Paul expresses his desire that the nation of Israel be saved because they are zealous for God, but they do not know God. Therefore, they do not understand God’s righteousness, and seek to make their own, thus failing to accept imputed righteousness. The reason that they failed to accept imputed righteousness by trying to make their own is that Christ brings the end of trying to earn righteousness by obeying God’s Law to those who believe in Him.
Beginning in verse 5, Paul explains how Scripture teaches that Christ brings believers to the point of ceasing to try to earn righteousness. The first way that it teaches it is that Moses wrote that those who practice law-based righteousness will live by that righteousness. However, no one can practice that righteousness. Therefore, attempting to earn righteousness is a futile act. However, Paul explains from the Old Testament that righteousness based on faith, or imputed righteousness, recognizes that Christ ascended into heaven, and that He died. Furthermore, these facts are in the believer’s mouth, or confession, and heart, or worldview. In addition, these facts are components of the “word,” or message, of faith that Paul was preaching: if one confesses with one’s mouth that Jesus is the Supreme Authority, or Lord, and believes that He rose from the dead, one will be saved. Then, Paul says that faith in Christ results in imputed righteousness, and confession results in salvation. Next, he summarizes what he has just said: whoever believes in Christ will not be disappointed. Finally, he explains that Jew or Gentile can be saved through faith.
In verses 14-15, Paul presents some possible questions that might arise from him explaining that the word of faith is the only message of salvation. In verse 16, Paul implies that Israel did hear the word of faith, or the good news, but that they did not believe it. As proof of this fact, he quotes Isaiah as asking God about who believed their report, implying that not all of Israel believed it. Therefore, Paul concludes that faith in Christ comes from spiritually hearing the message of Christ.
In verse 18, Paul explicitly says that Israel heard the good news. He proves this by quoting more Scripture. Then, Paul proves that Israel knew that the good news was preached to the rest of the world because Scripture says that God would make Israel jealous of the Gentiles, which includes a group of people that is not a nation, and a nation without understanding.
In verse 20, Paul proves that Gentiles were saved by quoting a Scripture that says they found God. Finally, he proves from Scripture that God offered the gospel to Israel, but they were disobedient to Him.