Why couldn't the whole Gentile world be brought under Judaism and the Law?
The whole Gentile world could not be brought under Judaism and the Law because the work of the cross was completed and finished. With that accomplished, God could pour out His grace on the whole human family, not just Israel, rather than requiring Gentiles to come under legalism and the Law.
Les explains that Israel continued to reject Christ in the early Acts period, and then God raised up another apostle and sent him to the Gentiles with the Gospel of the grace of God. Because the work of the cross was now completed and finished, God could pour out His grace on the whole human family, not only Israel. There was no way to bring the whole Gentile world under Judaism, legalism, and the Law. In Acts chapter 2, however, God was still dealing with Israel, as Peter says, "Let all the house of Israel know assuredly that God hath made that same Jesus whom you crucified, Lord and Christ."