When did God begin mandating Israel's separation from all the other nations?
God began mandating Israel's separation from all other nations during the Egyptian plagues, just before the fourth plague. In Exodus 8:22-23 He declared He would sever Goshen, where His people dwelt, and put a division between His people and the other people. That was the beginning of Israel's mandated separation.
Les explains that Peter's realization that God is no respecter of persons came after a long history in which God had separated Israel from the other nations. He goes back to Exodus 8, while the plagues were unfolding, at the end of the third plague and just before the fourth. The first three plagues fell on Goshen, where Israel dwelt, as well as the rest of Egypt. But before the fourth plague, God said in Exodus 8:22 that He would sever, separate, or set apart the land of Goshen in which His people dwell, so that no swarms of flies would be there, to the end that they might know He is the Lord in the midst of the earth. Verse 23 repeats that He would put a division between His people and thy people. Les says that was the beginning of God totally mandating that the nation of Israel be separated from all the other nations of the world, and that separation never left them.