The division between Jew and Gentile is as old as Judaism itself. In Ephesians 2, Paul tells us just how great that divide was. We were called "the uncircumcised" by those called "the circumcised", which is done in the flesh by human hands. We were without the Messiah, and excluded from the citizenship of Israel. We were without hope, and God's covenants were alien to us.
But woven throughout the Old Testament is a promise of hope. God didn't choose the Jews for any specific reason. They weren't special. No more loved, not owed special favor. In reality, we see that God's love extends to all of Adam's helpless race. And so, we see numerous promises that God intended to bring the Gentiles in, and in Jesus, He did exactly that.
Through Christ Jesus, the dividing wall of hostility between Jews and Gentiles is utterly demolished. Its very ashes are forgotten. Indeed, Paul, a Jew himself, shows a special passion for Israel, setting up this strange, self-fulfilling prophecy in which the salvation of the Gentiles is good for the Jews, and the salvation of the Jews is good for the Gentiles. God's love is completely unfathomable. He took a nation that constantly rebels against Him and used them to bless the entire world! And He doesn't use this as an excuse to cast off His people, whom He foreknew, as some in the Church erroneously claim. Rather, He uses the Gentiles to return the blessing back to the Jews. This God is our God, beyond worthy of any praise we could ever offer Him.