I will put a division between My people and your people. Tomorrow this sign will occur.
God, in the midst of the plagues He brings on Egypt, switches gears and the judgments become explicitly localized to the Egyptians. The preceding plagues may have been localized, but nothing is said about it, so there is no way to know for certain. What I know is that the swarms of flies are promised to be only in Egypt. but Goshen will remain untouched by them. This promise would be made all the more obvious by the fact that the Israelites were shepherds — the tail end of Genesis tells me that shepherds are loathsome to the Egyptians, hence the Israelites living in Goshen — and livestock attract flies. Flies will swarm Egypt, but will leave the one place you would expect to see them alone.
I take comfort in the knowledge that God is able to put a division between His people and those under judgment. Not only a division, but a stark contrast. I, like the whiny Israelites of old, may not be the best follower of God, yet God is still able to keep me as He kept them.
Father, thank You for the reminder that You set a distinction between those who are Your own and those who are not. Please etch this reminder in my heart now and in the days to come.