Why could God pour out His grace on the whole human family and not just Israel?
Because the work of the cross was completed and finished, God could pour out His grace on the whole human family, not just Israel. Israel rejected Christ and continued rejecting in early Acts, so God raised up another apostle and sent him to the Gentiles with the Gospel of the grace of God.
Les explains that he took the lady through the Old Testament, the Abrahamic Covenant, how Christ came to Israel as Messiah, how Israel rejected Him, and how Israel continued rejecting in early Acts. Then God raised up another apostle and sent him to the Gentiles with the Gospel of the grace of God. Les concludes it is logical: there was no way to bring the whole Gentile world under Judaism, legalism, and the Law, because with the work of the cross completed and finished, God could pour out His grace on the whole human family, not just Israel. But in Acts chapter 2, it is still God dealing with Israel at Pentecost, where Peter says all the house of Israel should know assuredly that God made that same Jesus whom they crucified Lord and Christ.