What did God do to protect the Israelites starting with the fly plague?
Beginning with the fly plague, God set apart the land of Goshen where Israel dwelt, decreeing no swarms of flies would enter it. He drew an invisible line around Goshen, putting a division between His covenant people and Egypt as a sign that He is the Lord.
Les explains that the first two or three plagues evidently struck the Jew as well as the Egyptian, but beginning with the fly plague God says He will sever or set apart in that day the land of Goshen wherein the Israelites were dwelling, so that no swarms of flies shall be there. Les says God was proving His point by drawing an invisible line around Goshen that even the insects would not cross. Verse 23 calls it a division between God’s people Israel and Pharaoh’s people Egypt, a sign set for tomorrow. Les connects this to Israel’s history of being a different, set-apart covenant people, and to Paul’s New Testament teaching in 2 Corinthians 6, with Romans 15:4 saying Old Testament things were written for our learning: God puts a line of demarcation between the believer and the world, and no one can serve two masters.
And I will sever in that day the land of Goshen, in which my people dwell, that no swarms of flies shall be there; to the end thou mayest know that I am the LORD in the midst of the earth.