What opportunity did Israel reject when they rejected the Messiah?
By rejecting the Messiah, Israel rejected the opportunity to be a Kingdom of Priests to the Gentile world—a go-between mediating God's blessing to the nations. This calling, offered at Sinai if they obeyed, was forfeited, though God continued His purpose by bringing Abraham's blessing directly to Gentiles through Jesus Christ.
Les explains that when Israel fell and rejected the Messiah, she also rejected the glorious opportunity of being a Kingdom of Priests to the Gentile world. At Sinai in Exodus 19, God told them that if they were obedient, He would make them a Kingdom of Priests, and every Jew could have been a go-between between the Gentile world and God. Les says they blew it and dropped the ball, but God did not stop. Now Paul writes in Galatians 3:14 that Gentiles receive the blessing of Abraham directly through the Lord Jesus Christ, not through Israel, so we do not have to wait for Israel to get her act together or for a Jew to bring us knowledge of God. God went straight to the pagan world with salvation without Israel as a go-between.