What was the mystery about Israel's blindness while God called the Gentiles?
The mystery was that while God spent roughly 1,900 years calling out the Gentiles, Israel would be in spiritual blindness. This blindness was only in part and temporary, lasting until the fullness of the Gentiles—the body of Christ—is brought in; then Israel's blindness ends and spiritual relationship with Jehovah resumes.
Les explains that this was a secret kept in the mind of God: while God would spend some 1,900 years calling out the Gentiles, Israel would be in spiritual blindness. This was a mystery now revealed, not found elsewhere in Scripture. Isaiah had said their table would become a snare and that they would be blind with ears that would not hear, but he did not connect it to a period when God would turn to the Gentiles. The blindness was in part, not forever, and would last only until the fullness of the Gentiles—the body of Christ—was brought in. When the last person is saved and the church is removed at the rapture, God will pick up where He left off with Israel, the Jew, and their spiritual blindness will end as they again have a spiritual relationship with Jehovah.