Romanos 11:25

A "mystery" in the New Testament refers to a truth previously unknown but now revealed. In the "mystery religions" of the Greco-Roman world, initiates received secret information that was unknown to non-initiates. A modern counterpart is the "secret societies" (e.g., the Masons, the Eastern Star, et al.). That revelation in this case was that Israel (ethnic Jews) would experience a partial hardening from God until the full number of elect Gentiles would be saved. God’s plan to put the nation of Israel aside temporarily should not make Gentile believers think too highly of themselves. God designed this plan to display His own glory.

We must be careful not to equate the modern State of Israel with the Israel spoken of in the Bible. Biblical Israel was a sovereign nation among nations in the world that lost its sovereignty when Nebuchadnezzar destroyed Jerusalem in 586 B.C. Whereas some Jews today have organized the State of Israel, God has promised that He will yet cause the great majority of Jewish people to believe on His Son and return to the Promised Land as believers in Him. This will happen when Jesus Christ returns to the earth. He will then reestablish Israel as the people of God and reign over them as their Davidic King (cf. Zechariah 12-14). The present State of Israel is presently not enjoying the abundant blessings God promised to bring on Israel when Christ returns.

The "fullness of the Gentiles" (NASB) refers to the "full number of the Gentiles" (NIV; cf. Romans 11:12; Luke 21:23-24; Acts 15:14). When all the Gentiles whom God has chosen for salvation during the present age of Jewish rejection (setting aside) have experienced salvation, God will precipitate a revival of faith among the Jews. Even though some Jews trust Christ now, God is not presently working through them as Israel as He will in the future (i.e., in the Millennium), after multitudes of them turn to faith in Christ. He is now working through the church.