Romans 9:32
Wherefore? Because they sought it not by faith, but as it were by the works of the law. For they stumbled at that stumblingstone; Wherefore? Because they sought it not by faith, but as it were by the works of the law. For they stumbled at that stumblingstone;
The stumblingstone is Christ Himself, and Israel stumbled at Him because they sought righteousness not by faith but as if by works of law, refusing to receive Him as personal Saviour. To refuse the truth and mercy of God turns that very truth and mercy into a stumbling block, so that those who reject the light and reproof He gives trip and fall and are left to go on in a downward course.
Professed believers in Christ stumble in the same way when they separate Christ and the law and make faith a substitute for obedience. They declare that believing alone is enough, that the imputed righteousness of Christ keeps the law for them, and that they are under no obligation to obey the law of God. The garments of Christ's righteousness are not a cloak for wrongdoing, and no one is saved while breaking God's law, so to teach that His death saves men while they continue in transgression makes Him a stumbling block. The same picture applies to helping those who have stumbled, viewing their difficulties as stones blocking the path that should be taken away by loving effort, with human and divine help united, in order to recover them.