How do we know that God's calling of the Gentiles was part of His eternal plan?

In Ephesians 3:6, Paul states that the inclusion of Gentiles into the promise of God was a mystery hidden in God from the beginning but now revealed through the gospel. This counters the erroneous idea that God's plan shifted after the rejection of the Jews. Instead, Paul emphasizes that the calling of the Gentiles to be fellow heirs was always part of God's sovereign plan, demonstrating His grace and mercy toward all nations. Such a divine purpose underscores the unity of the church as one body of believers in Christ.