Israel is the Old Testament Church and the Church is the New Testament Israel.

And certain men came down from Judea and taught the brethren, "Unless you are circumcised according to the custom of Moses, you cannot be saved." Therefore, when Paul and Barnabas had no small dissension and dispute with them, they determined that Paul and Barnabas and certain others of them should go up to Jerusalem, to the apostles and elders, about this question. Acts 15:1-2

Having made a case that Christianity is not exclusively a Jewish Religion, neither is it exclusively a Gentile religion. The Bible doesn’t have an Old Testament about God’s dealing with the Jews and Israel, and a New Testament about God’s dealings with the Gentiles and the Church.

Rather, the Bible isn’t primarily about us. It is primarily about God and His plan to save both Jews and Gentiles throughout all of human history for His eternal glory.

According to God’s plan, in the Old Testament Israel was to be a light to the Gentile nations via keeping God’s law. Even Israel’s failure to understand and do this was a part of God’s plan to reveal in the New Testament that the only way to be right with God is through faith in God’s Son, Jesus Christ. So when Jesus came in the New Testament, He wasn’t plan B. Plan A has always been about Jesus—in the Old Testament foretold, and in the New Testament revealed.

Israel is the Old Testament Church and the Church is New Testament Israel. One people, in Christ, regardless of race or ethnicity. The Church is not instead of Israel, but the completion of Israel. The Church has not replaced Israel but is the fulfillment of God’s plan that began with Israel, but now includes all peoples.