What shall we say, then? That Gentiles who did not pursue righteousness have attained it, that is, a righteousness that is by faith; but that Israel who pursued a law that would lead to righteousness did not succeed in reaching that law. Why? Because they did not pursue it by faith, but as if it were based on works. They have stumbled over the stumbling stone, as it is written,
“Behold, I am laying in Zion a stone of stumbling, and a rock of offense;
and whoever believes in him will not be put to shame.”
With this simple exposition of Isaiah 28:16, Paul lays out the clear explanation of just why the Gentile world was turning to the gospel, even as Jews were tending to reject Jesus. Israel had been placed as God's light to the Gentile world. They had been given the Law, the Prophets, and the Savior Jesus Christ to share with the world. Yet they had neglected that faithful responsibility for the most part, caught up in the soul-destroying mess of legalism that instead rejected God's grace that had been poured out to them in Jesus. The gospel in Jesus' life, death, and resurrection was Israel's stumbling stone laid by God in Zion. The resulting works-based religion kept them from embracing the faith necessary to believe the gospel.
Works righteousness is shown to be a serious gospel killer. Paul's teaching was that it kept Israel as an ethnic group largely ineffective in pleasing God. They chose, as a group, to follow works-based righteousness rather than faith-based repentance, belief, and obedience from the heart. They failed at keeping the law by works. They could not accept Jesus by faith as long as they were caught in the trap of their works.
Jesus calls me to repent and believe the saving message of His atoning death for my sins. He calls me to believe that in His resurrection I have new and eternal life. And only after I have done this can I obey God's Word and live in the holiness provided for me in Jesus. I am called then to a life of faith-filled repentance and belief as I keep turning from sin to obey God's truth and be more like Jesus. Even then it is only possible because the Holy Spirit has regenerated me and empowered me to do so. I am a sinner, saved by grace, not by works so that I can only boast in Jesus!