What would have happened if Israel repented after Peter's Acts 3 sermon?
If Israel had repented of national sins and believed who Jesus was, God would have brought in the tribulation; it could have run its course, Christ could have returned, and Israel could have had the kingdom. Then Peter and the eleven could take the gospel to the Gentiles.
Les teaches that God was still dealing with the nation of Israel while Peter and the eleven remained in Jerusalem. If Israel would repent of their national sins and believe who Jesus was, God would have brought in the tribulation, it could have run its course, Christ could have returned, and Israel could have had the kingdom. Only when that was fulfilled could Peter and the eleven take the gospel of their God of Abraham to the Gentile world. The twelve did not leave Jerusalem, even under pressure, because it was to Jerusalem and the Mount of Olives that their king was going to imminently return. Zechariah 8:20-23 foretold many peoples and strong nations coming to seek the Lord in Jerusalem.