What does Romans 11:18 mean?
This verse concludes the "then" part of an if/then statement begun in the previous verse. Paul has said to Gentile Christians that they have been grafted into the tree of God's special relationship with Israel. Now, he says Gentiles must not be arrogant toward the Jewish "branches."Perhaps some of the Gentile Christians of Paul's day were being harsh or dismissive about the lack of faith in Christ among most Jewish people. Since some in Israel's leadership were persecuting Christians, it's understandable that some Christians would lash back against them. Even today, there are those who attempt—falsely—to justify antisemitism through Israel's rejection of Jesus.
Paul doesn't want these Gentile believers to miss an essential truth, however. They are the ones being "artificially" inserted into a relationship with God. This is happening only by His grace and through faith in Christ. These Gentiles are not nearly the first to have a special relationship with God. They are not the root: the vital, life-giving source of the plant, taken from the Greek term rhiza.
In the flow of this passage, the "root" is likely the patriarchs of Israel who received God's promises. Those roots, and the trunk which now grows from it are what supports these new Gentile Christians. They, the grafted-in branches, do not give life to the rest of the tree. Humility about Israel's special and ongoing relationship with God is the only response that makes sense for non-Jewish believers.