What then shall we say? That the Gentiles, who did not pursue righteousness, have obtained it, a righteousness that is by faith; but the people of Israel, who pursued the law as the way of righteousness, have not attained their goal. Why not? Because they pursued it not by faith but as if it were by works. They stumbled over the stumbling stone. As it is written:
“See, I lay in Zion a stone that causes people to stumble
and a rock that makes them fall,
and the one who believes in him will never be put to shame.” (Romans 8:30-33).
Simply put, the Jews stumbled in that they pursued a righteousness by works rather than a righteousness by faith. But the deeper question is in what sense was this a stumbling block?
A stumbling block is something that is not obvious, causes difficulty and produces hesitation. The Jews believed that they could obtain righteousness by obeying the Mosaic Law. They believed that God gave the Law so that they might attain righteousness through it. As sensible as this at first seems, they were badly mistaken.
The Law was given not to show them how righteous they could be but to reveal just how unrighteous they actually were. As Paul wrote earlier,
Therefore no one will be declared righteous in God’s sight by the works of the law; rather, through the law we become conscious of our sin...There is no difference between Jew and Gentile, for all have sinned and fall short of the glory of God (Romans 3:20,22-23).
The Law was supposed to cause Israel to humble themselves before the LORD, keep them continually repentant before Him, and lead them to simply trust in God's provision alone for mercy, grace and salvation.
We are told that it was not for lack of zeal that the Jews were unsaved. It was their lack of knowledge. It was their failure to recognize that the way of salvation was not by their own efforts but through faith in God's gracious provision of Christ. So then, just after Paul wrote of the stumbling block, he wrote,
Brothers and sisters, my heart’s desire and prayer to God for the Israelites is that they may be saved. For I can testify about them that they are zealous for God, but their zeal is not based on knowledge. Since they did not know the righteousness of God and sought to establish their own, they did not submit to God’s righteousness. Christ is the culmination of the law so that there may be righteousness for everyone who believes (Romans 10:1-4).
So then, in what way did the Jews stumble? They thought that God gave them the Law as a way for them to become righteous rather than to show them their utter sinfulness and helplessness to save themselves. Zealously grasping this mistaken notion, they failed to recognize the true way of righteousness, which came by faith in God's free gift of salvation in Christ.