When did God start requiring His people to be separate from the world?
God began requiring separation from the world at the very start of His dealings with Abraham. Abraham could not remain in Ur among pagan, idolatrous relatives; separation had to occur before God could work with him. This principle continued, as God later severed Goshen from Egypt during the plagues.
Les teaches that separation did not begin with Israel at Sinai or with the Exodus; it started "with day one" in God's dealings with Abraham. God could not deal with Abraham while he was still in Ur amid all his pagan, idolatrous relatives, so there had to be a separation before God could work with him. Later, after Lot separated from Abram in Genesis 13, the Lord spoke to Abram again. In Exodus 8, during the plagues on Pharaoh's Egypt, the first three plagues fell on Israel as well as Egypt, but God said He would sever or separate the land of Goshen where His people dwelt, so no swarms of flies would be there, proving He is the Lord in the midst of the earth. Thus the requirement of separation from the world began at the outset and continued.