Why did Israel's rejection of the Messiah open the door for the Gentiles?
Israel’s rejection and crucifixion of the Messiah, though foreknown by God, opened the windows of heaven for the Gentiles. God saved Saul of Tarsus by grace and sent him far hence to the Gentiles, while Israel entered spiritual blindness. Formerly pagan Gentiles then swarmed to the Gospel with spiritual hunger.
Les explains that Israel could have had the King and the Kingdom, and could have evangelized Gentiles, but this was hypothetical; in God’s foreknowledge it could not happen, because He had to die. Israel rejected and crucified Him and continued rejecting Him in early Acts, even though Peter and the eleven preached. When Israel proved they would not believe, God raised up Saul of Tarsus—one who hated Jesus of Nazareth—and saved him by grace, the epitome of grace. He told Saul before he even got out of Damascus that He would send him far hence to the Gentiles. Israel would go into spiritual blindness, but Saul would go to the Gentiles. Because Israel rejected the Old Testament promises, her Messiah, and her King, it opened the windows of heaven for Gentiles whose forefathers were pagan, steeped in idolatry, the occult, mythology, or whatever. Israel alone had the true knowledge of the true God, but God did something totally different: He sent this apostle among pagan Gentiles, and they began to swarm to the Gospel with spiritual hunger.