John Calvin Commentary Romans 11:18

"glory not over the branches: but if thou gloriest, it is not thou that bearest the root, but the root thee." — Romans 11:18 (ASV)

But if thou gloriest, thou bearest not the root, etc. The Gentiles could not contend with the Jews respecting the excellence of their race without contending with Abraham himself. This would have been extremely unbecoming, since he was like a root by which they were supported and nourished. As unreasonable as it would be for the branches to boast against the root, so unreasonable would it have been for the Gentiles to glory against the Jews (that is, with respect to the excellence of their race); for Paul would have them always consider from where their salvation originated.

We know that after Christ, by His coming, pulled down the partition-wall, the whole world partook of the favor that God had previously conferred on the chosen people. It therefore follows that the calling of the Gentiles was like an ingrafting, and that they did not otherwise grow up as God’s people than as they were grafted into the stock of Abraham.