What did the division between Israel and Egypt symbolize?
The division symbolized God's setting apart of Israel as His covenant people from Egypt and the pagan nations, showing they were different and called to have nothing to do with surrounding Gentiles. It also pictures the line of demarcation God places between the believer and the world, since no one can serve two masters.
Les teaches that God drew an invisible line around Goshen so even the insects would not cross, proving His point and showing the Egyptians that He is the Lord in the midst of the earth. The division was a setting apart of the children of Israel from those of Egypt. All through Israel's history they were told, "You're different. You are to be a set apart people. You are the covenant people. You are to have nothing to do with those pagan Gentiles all around you." Les then gives a New Testament analogy: just as God put a separation between the children of Israel and Egypt, God puts a line of demarcation between the believer and the world, and God says you cannot serve two masters.
And I will put a division between my people and thy people: to morrow shall this sign be.