Why did Israel not believe? Paul answers this question. They did not have faith. This is a continuation of Romans chapter 9.
Israel Did Not Believe
30 What will we say then? [We will say] that the Gentiles who didn’t pursue [God’s standard of] righteousness have obtained righteousness, even the righteousness that is because of faith.
[Righteousness is because of faith. The Pulpit commentary comments on this verse: Hitherto the apostle has viewed his subject from the side of the Divine will and purpose. He now views it from the side of human responsibility. The rejection of the Jews is now attributed, not to God’s purpose to reject them, but to their own fault, in that they would not accept God’s terms].
31 But Israel, who pursued the law of righteousness, has not reached [the goal] of the law of righteousness.
[Israel failed because they believed they could keep the law, and if they did sin, an animal offering forgave sin. Israel believed they did not need a Messiah to deliver from sin, but only a Messiah who would deliver from enemies. As the hymn reminds us:
Not the labours of my hands
Can fulfil thy law’s demands;
Could my zeal no respite know,
Could my tears for ever flow,
All for sin could not atone:
Thou must save, and thou alone].
32 Why? Because [they didn’t seek it] by faith but as if it were by the works [of the law], for they stumbled at the stumbling stone.
[Joseph Benson writes: For they stumbled at that stumbling-stone — Which lay in their way. This is an allusion to one who, running in a race, stumbles on a stone in his way, and, falling, loses the race. The word “at” is not explicitly in the Greek but is understood by the dative case. The “stumbling stone is Jesus Christ (1 Peter 2:4-8)].
33 As it is written, “Look, I lay in Zion a stumbling stone and a rock of offense, and whosoever believes on him will not be ashamed.”
[Isaiah 28:16 (Septuagint) and Isaiah 8:14. Compare Romans 10:12-13. It is not works (Romans 2:13 and Romans 3:20) but faith that saves, forgives, and reconciles us to God (Romans 3:28, Romans 10:9, and Galatians 2:16].