What shall we say then? That Gentiles, who did not pursue righteousness, have attained to righteousness, even the righteousness of faith; but Israel, pursuing the law of righteousness, has not attained to the law of righteousness. Why? Because they did not seek it by faith, but as it were, by the works of the law. For they stumbled at that stumbling stone. As it is written: "Behold, I lay in Zion a stumbling stone and rock of offense, And whoever believes on Him will not be put to shame." (Romans 9:30-33)
The stone is Christ. The stumbling stone separates the sincere in heart towards God from the insincere. The unbelieving do not have a true heart for what is right or for truth and so they stumble on the stumbling stone, they stumble at Christ.
For with the heart one believes unto righteousness, and with the mouth confession is made unto salvation. (Romans 10:10)