What would have happened if Israel had believed that Jesus was their King?
If Israel had believed Peter's preaching that the crucified One was their King, the King would have returned, the Kingdom would have been established, and all Old Testament promises—including the blessing going to the Gentiles—would have been theirs. However, God in His foreknowledge knew they would reject it.
Les teaches that the twelve apostles were eagerly expecting the King to set up His Kingdom immediately. Peter preached to the nation of Israel that the One they crucified was indeed their King. If they could just believe it, the King would yet return, and they could have the Kingdom with all the Old Testament promises of going to the Gentiles. But God in His foreknowledge knew they wouldn't do it; they would reject it. This hope was tied to Jesus' promise that they would sit on twelve thrones ruling the twelve tribes of Israel.