Exodus Chapter 8 Verses 20-23 

:20 “And the LORD said unto Moses, Rise up early in the morning, and stand before Pharaoh; lo, he cometh forth to the water; and say unto him, Thus saith the LORD, Let my people go, that they may serve me. 21 Else, if thou wilt not let my people go, behold, I will send swarms of flies upon thee, and upon thy servants, and upon thy people, and into thy houses: and the houses of the Egyptians shall be full of swarms of flies, and also the ground whereon they are. 22 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. 23 And I will put a division between my people and thy people: to morrow shall this sign be.”

“And the LORD said unto Moses, Rise up early in the morning, and stand before Pharaoh; lo, he cometh forth to the water; and say unto him, Thus saith the LORD, Let my people go, that they may serve me.”  Here, we find that the LORD knew beforehand the place and time that Pharaoh would come the next day.  This illustrates the omniscience of God.  Also, we see that before every judgment that the LORD warned Pharaoh to let God’s people go that they may serve him.  The LORD’s messengers were to faithfully deliver the message as the LORD gave it to them.  They were not to take anything from the message and they were not to add anything to the message. 

“Else, if thou wilt not let my people go, behold, I will send swarms of flies upon thee, and upon thy servants, and upon thy people, and into thy houses: and the houses of the Egyptians shall be full of swarms of flies, and also the ground whereon they are.” 

Flies are filthy disease carrying insects that are loathed among all people.  They are filthy and they leave their filth behind them when they leave.  The depraved nature of man is like the filthy flies.  The description of our depraved nature in Rom. 3:9-18 shows us just how filthy our nature is and how we leave our filth behind us: Rom. 3:9 “What then? are we better than they? No, in no wise: for we have before proved both Jews and Gentiles, that they are all under sin; 10 As it is written, There is none righteous, no, not one: 11 There is none that understandeth, there is none that seeketh after God. 12 They are all gone out of the way, they are together become unprofitable; there is none that doeth good, no, not one. 13 Their throat is an open sepulchre; with their tongues they have used deceit; the poison of asps is under their lips: 14 Whose mouth is full of cursing and bitterness: 15 Their feet are swift to shed blood: 16 Destruction and misery are in their ways: 17 And the way of peace have they not known: 18 There is no fear of God before their eyes.” 

The flies were not just upon the land of Egypt, but were also in the houses of the Egyptians.   

“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. 23 And I will put a division between my people and thy people: to morrow shall this sign be.”  The doctrine of God’s sovereign election is taught here and that God makes a separation in judgment.  The children of Israel were spared from this point on in the judgments of God upon Pharaoh and upon the Egyptians.  Moreover, God made it abundantly clear to Pharaoh that he had made such a separation.