And again it is said, Rejoice, O Gentiles, with his people.” (ESV)
The Apostle Paul is quoting from the Torah, the Old Testament Law. The Law was such a special thing to the people of Israel. They would hear the Law read from childhood. Their parents were instructed to teach their children the books of the Law every day. The Law was a distinctly Jewish commodity. It was not understood that the Law was for Gentiles.
Christ comes into the world and everything changes. He was born as a Jew. He would have heard the Law read to him all the time as a child. He would have followed the Law at his earliest age. Joseph and Mary would have faithfully taught him the Law in every situation they could as Jesus grew up. Jesus was a Jew.
The Jews had no idea who Jesus really was. They crucified the very Son of God. He was their Messiah. They had him put to death. While this all seems disastrous it was actually part of God's perfect plan. Jesus fulfilled the Law perfectly in his body, sacrificed himself for all mankind, and made a new way for all people to be made right with God.
Rejoice, O Gentiles, with his people. The Gentiles were included with the people of faith, the Jews. There is no longer a distinction between the two groups of people. There were many Jews who had placed their faith in Christ who still struggled with the idea that the Gentiles were going to be worshiping alongside them. It was all part of God's perfect plan.
Take a few moments and rejoice! Give praise to God that he made a way for you to be included with all of God's people.