Adam Clarke on Romans 11:18

Boast not against the branches - While you are ready to acknowledge that you were included in the covenant made with Abraham, and are now partakers of the same blessings with him, do not exult over, much less insult, the branches, his present descendants, whose place you now fill up, according to the election of grace: for remember, ye are not the root, nor do ye bear the root, but the root bears you. You have not been the means of deriving any blessing on the Jewish people; but through that very people, which you may be tempted to despise, all the blessing and excellencies which you enjoy have been communicated to you.