In this passage, Paul is still answering the question from verse 11, "Is Israel's fall a permanent destruction?" The answer is no, and he uses the illustration of an olive tree to make an application point, especially for Gentile Christians. Gentiles, through faith in Christ, are like wild olive tree branches that have been grafted to another tree. But God first broke off many natural branches, ethnic Israelites, for not believing that Jesus is the Messiah. Their "loss" is the Gentiles "gain." But this should by no means lead Gentiles Christians to be arrogant or disdain ethnic Jews, either in Paul's day, or ours. It is all by the grace of God that any receive eternal life in Christ alone, both Jew and Gentile.