What does Romans 9:33 mean?

Why did the people of Israel not become righteous and acceptable before God by following the law of Moses? That's the question Paul is answering. He has said that the Israelites sought to be made righteous, and earn God's mercy, by their works. They approached God through a work-and-earn approach, rather than by faith. Paul concluded the previous verse by writing that they—Israel—have stumbled over the stumbling stone.

The idea of a "stumbling stone," as mentioned here is an odd thing. Who would deliberately place a rock for people to trip over? Paul's quotes from Isaiah in this verse shows that the stone is tripped over because it is rejected, rather than being received by faith.

First Paul quotes from Isaiah 8:14, where the prophet describes the Lord as a rock of offense and a stone of stumbling to all of Israel. Paul is insisting that this is not a new idea—that the Jewish people would struggle to accept the idea that righteousness is found only through faith in Christ.

Next, Paul quotes from Isaiah 28:16, where that stone, Christ, is described as the sure foundation, the precious cornerstone of God's work on earth. The part Paul quotes directly is that whoever believes in him will not be put to shame. In more literal terms, such people will not be shown to be foolish for believing in Christ. This, symbolically, is the stone that the Israelites have tripped over, Paul writes, because they refused to believe in Christ, wishing instead to prove their righteousness by their own works.