What does Romans 11:21 mean?
Paul has been warning the Gentile Christians not to be arrogant and prideful about their place in God's family. They should not be puffed up about their position on the symbolic olive tree of those who belong to God through faith. He has specifically told them not to be arrogant toward the unbelieving Jewish people: those who had rejected Christ and have been removed from the tree.The only difference between the Jewish people who have been pruned by God and the Gentile Christians who have been added is faith in Christ. The Jewish branches that were pruned were taken off because of their refusal to believe. The Gentile Christians were added because they believed. Faith in Christ is the only difference.
Now Paul writes that if God was willing to break off the natural branches, those Jewish people who were genetic descendants of Abraham, He certainly won't spare Gentiles who refuse to believe in Christ. Faith in Christ is their only hope of remaining connected to the tree.