John Calvin Commentary Romans 11:17

"But if some of the branches were broken off, and thou, being a wild olive, wast grafted in among them, and didst become partaker with them of the root of the fatness of the olive tree;" — Romans 11:17 (ASV)

And if some of the branches, etc. He now refers to the present dignity of the Gentiles, which is simply to be branches; these, having been taken from another, are grafted into some noble tree. For their origin was, as it were, from a wild and unfruitful olive, as nothing but a curse was to be found in their whole race. Whatever glory, therefore, they had was from their new grafting, not from their original stock. Therefore, there was no reason for the Gentiles to glory in their own dignity in comparison with the Jews.

We may also add that Paul wisely mitigates the severity of the case by not saying that the whole top of the tree was cut off, but that some of the branches were broken, and also that God took some here and there from among the Gentiles, whom He set in the holy and blessed trunk.