How do we know the Church is not separate from Israel?

The idea that the Church is distinct from Israel is countered by multiple passages in the New Testament. For instance, Ephesians 3:4-6 reveals the mystery that Gentiles are fellow heirs and members of the same body with Jews. Moreover, 1 Peter 2:9 applies Old Testament promises to the Church, showing that all believers, whether Jew or Gentile, are now part of a chosen race and holy nation. This incorporation highlights that Israel's promises now extend to the Church, reaffirming that there is no separate plan for ethnic Israel based on the New Covenant.