What mystery about Jews and Gentiles was hidden in the rest of Scripture?
The mystery hidden from the rest of Scripture was that Jews and Gentiles would be brought into the Body on equal ground. For personal salvation, a Jew is on the same level playing field as a Gentile, not in a place of special privilege, because both are sinners who need salvation.
Les explains that Paul is revealing one of the mysteries that belong to the volume of truth he calls the revelation of the mysteries. There are all kinds of them. This mystery was unknown to the rest of Scripture: Jews and Gentiles would be brought into the Body on equal ground. Les respects Israel's covenant position as God's favored nation, but he stresses that a Jew today experiences salvation on the same level playing field as Gentiles. A Jew is not specially privileged in personal salvation; he is a sinner who needs salvation as much as a Gentile. Yet Israel must be in the Land for the culmination of the prophetic Scriptures. Les illustrates with a young Jewish listener who noted that if all Jews were saved, they would leave at the rapture and no one would be left to fulfill prophecy. Israel has to be in the Land for the final seven years, because all prophecy is directed to that. Removing them would make God's Word fall apart, and God will not let it happen.