But I say, Did not Israel know? First Moses says,
I will provoke you to jealousy by them that are no people, and by a foolish nation I will anger you.
Read Chapter 10
Paul means here that of course Israel knew…. They all heard but they did not all believe. The jealousy of the Jews arose from their envy at seeing a people which earlier had been without God and barbarous claim the Jewish God as their own and receive the promise which had originally been made to the Jews…. Nothing destroys a man so much as jealousy, which is why God made it the avenger of unbelief, because that is a great sin. Commentary on Paul’s Epistles.
By calling a people “foolish” Paul explained what he meant by “those who are not a people,” viz., a foolish people ought not to be called a people at all. But he says that the Jewish people will be angered by the Gentiles’ faith, because they have received what the Jews have rejected…. Even though entire peoples were foolish idol worshipers, they nevertheless put away their paganism by believing. Thus Paul said: “If a man who is uncircumcised keeps the precepts of the law, will not his uncircumcision be regarded as circumcision?” Thus he means: “I will make you jealous of those who once were not a people but were made a people,” because although they were once a foolish idolworshiping people, they put aside their paganism through their faith in Christ.
Whence it was said to them by Moses, "I will provoke you to jealousy by them that are not a people; and I will anger you by a foolish nation, that is, by one that has become disposed to obedience.".
And by Isaiah it is said, "I was found of them that sought Me not; I was made manifest to them that inquired not after Me"
A people whom you have always despised, as not being worthy to be numbered amongst mankind, I will make my beloved people. I will enrich with my graces those whom you have contemned; I will give them the grace of adoption, thus to stir you up to jealousy and rage. And in effect, what greater rage ever was there, than that of the Jews against the converted Gentiles? In persecution, the Jews were always most busy. Thus were the Jews plainly informed of the truth of the gospel, by the Gentiles converted before their face. (Calmet)
Israel ought to have known even on the basis of Moses alone how to tell who were the true preachers, not merely from the fact that they believed, nor from the fact that the preachers spoke of peace, nor from the fact that they brought glad tidings of good things, nor from the fact that the word was sown in all the world, but from the ironic fact that their inferiors, the Gentiles, were receiving greater honor than they had.