What would have happened if the prophesied King and kingdom had come for Israel?
If Israel had believed and obeyed, the prophesied King would have returned soon after His ascension and set up His kingdom, and Israel would have evangelized the Gentiles. However, this was hypothetical because it depended on Israel's obedience and belief in Jesus as the Christ.
Les explains that God the Father invited the Son to sit at His right hand until His enemies were made His footstool (Psalms 110:1). But according to the Old Testament, He would not have stayed there long before coming right back to set up His kingdom—if Israel had been obedient and believed Jesus was the Christ. This was the whole thrust of Peter and the eleven preaching in Acts: the one they crucified was the Christ, their King, and He could yet come and be their King. All these things were prophesied to fall into place for the coming of the King and kingdom, and then Israel could have evangelized the Gentiles. But in reality, Israel rejected it in unbelief, and God instead turned to the Gentiles with the age of grace.