Why did Israel lose her place as the go-between between God and the Gentiles?
Israel lost her place as go-between because she rejected the Messiah, and in doing so rejected her opportunity to be a Kingdom of Priests to the Gentiles. Though Exodus 19 offered that role if obedient, she failed. Consequently, God bypassed Israel and brought salvation directly to Gentiles through Jesus Christ.
Les explains that Israel fell out of the picture when she fell. When she rejected the Messiah, she also rejected the glorious opportunity of being a Kingdom of Priests to the Gentile world. In Exodus chapter 19, God told them at Sinai that if they would be obedient, He would make of them a Kingdom of Priests. Every Jew could have been a go-between between the Gentile world and their God. But they blew it and dropped the ball. However, that did not stop God, because now Paul can write that the Gentiles have received the blessing of Abraham, not through Israel, but directly through the Lord Jesus Christ. God went straight to the pagan world with the message of salvation without Israel as a go-between. We do not have to wait for Israel to get her act together, nor for a Jew to come to our community to give us the knowledge of their God.