Not be ashamed. Romans 9:30-33 V.30. Israel failed to obey their own God-given law, which was given to bring us to Christ. They claimed to pursue the law by their works instead of by faith. Most of those who will be rejected will be because they try to earn God’s favor by their works instead of accepting what He has already provided by faith. It may seem to some people unfair that Gentiles who were, and many still are, idolaters and those who weren't even interested in righteousness are ones whom God saves in sovereign in electing grace. The Israelis who were pursuing the law earnestly to become righteous have not succeeded in attaining God's salvation.
V.31. The problem was that the Jews were pursuing the law and were seeking to attain a righteous standing before God by their own self-efforts and couldn't rise to the standards of the law. Obedience to the law is good, but seeking to obligate God to accept them and put Him in debt is absolutely wrong. Their object of trust was themselves, and when Christ came, instead of repenting of their sin and failure and turning to Christ and trusting Him, they turned on Him and killed Him.
The purpose of the law was to bring people into a right relationship with a holy God by showing them their sins and pointing them to Christ for forgiveness when they put personal faith in Him alone. The One they stumbled over was the One who was promised in the Garden of Eden. He was the Promised One to Abraham, David, and all the people of faith who looked beyond the law to Him - the Messiah. It was actually in Zion where He laid the basis of our faith. The Rock of offense the Jews stumbled over is the Chief Corner Stone to the Church.
V.32. To seek the righteousness of God by works is a form of an insult. The sin-polluted works of men only serve to make the gift of God's salvation by grace seem of lesser value to those who hold to that "Jesus plus ---" teaching. The Jews would not believe in the Lord Jesus Christ because He didn't meet their expectations of the One coming as the conquering King. Even today, some people expect God to overlook their sins and accept them as they are. Others think they can manipulate God into accepting them because of who they are or what they do. Anyone who refuses to humble themselves and put their faith in Christ alone will be put to shame.
V.33. Because they wanted to be accepted by God through their own good works, they rejected Christ and became a stumbling stone to themselves in their search for salvation on their own terms. Salvation by grace is offensive to many because it means we have a need we cannot meet. Faith requires acknowledging personal guilt for sin and honest humility in repentance when we face our helplessness to come to God by our own efforts.
We must accept the truth of God's electing grace. No one will ever meet the standard of perfection for salvation. Meeting the demands of holiness, which the law gives, is impossible through our efforts. Through the understanding given by the preaching of the Gospel, we have to believe that Christ is the Savior of sinners and anyone who trusts in Him will not be put to shame. Obedience to Him means we put our will aside and face our dependence on and need of Him.
We are, by nature, lost because of sin and, as guilty sinners, cannot save ourselves from being children of wrath. Because we are of the world, God had to make His move to us, which He graciously did. He gave us His great salvation in mercy and love, full and free to "whosoever will may come." He desires "all men be saved and come to the knowledge of the truth." His offer of salvation still holds, and we can invite all who hear the Gospel to come to Him and be saved.
God does not compromise His holiness and righteousness because that would be inconsistent with His nature. But in grace and love, His mercy reaches us with forgiveness through Christ. Salvation is not because of any works we may do but is all of God Himself. These parallel truths make our acceptance by God possible.