Why? Because they sought it not by faith, but as it were by the works of the law. For they stumbled at that stumbling stone;
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The Jews rejected faith, which as I have said is the fulfillment of the law, and instead claimed that they were justified by works, that is, by the sabbath, the new moons, circumcision and so on. They forgot that Scripture says that “the just shall live by faith.” Commentary on Paul’s Epistles.
Paul calls the Lord Christ a stumbling stone because those who did not accept the new covenant in him stumbled over him and by their unbelief fell from the grace of justification which was given to men through him. .
Of the passion itself should be figuratively set forth in predictions; and the more incredible (that mystery), the more likely to be "a stumbling-stone".
and knowing how to bear infirmity: "to wit as having been set by the Father "for a stone of offence"