“Remember it is not you who support the root, but the root that supports you” (v18). Paul cautioned the Gentile-background believers from becoming arrogant and feeling superior to the Jews, God’s ancient chosen people. The Israelites’ arrogance and unbelief led to their rejecting the Messiah; Paul warned that this could happen to any group which relied on their “insider status” with God (v21-22). God does not need our support or defense against “outsiders”; He freely offers salvation, and He is the root that supports us all.
Our connection to the “root,” Christ Himself, is not through culture or language or birth, but through faith. We were all sinners, and we have all received mercy (v32).