Why did God send the plagues against the gods of Egypt?
God sent the plagues to execute judgment against all the gods of Egypt. Every plague was directed against one of Egypt's gods, proving their pagan worship had nothing to do with Him. He could destroy them at will, showing Israel's God alone was the true God.
Les teaches that God said He would pass through the land of Egypt, smite all the firstborn both man and beast, and against all the gods of Egypt execute judgment. Les points out that every one of the plagues was directed against one of the gods of Egypt. By this, God proved that Egypt's pagan worship had nothing to do with Him and that He could destroy them at will. Les also stresses that ever since the Tower of Babel, the whole human race until Abraham was saturated in paganism and polytheism, the worship of many gods. When Israel comes on the scene as His separated, different covenant people, they are the only people on earth with a knowledge of the one true God. At that time, everyone else in the populated world, except those coming out of Abraham, Isaac, and Jacob, were pagan worshipers of many gods. So the plagues were God's judgment on Egypt's gods and a demonstration that He alone is the true God.
For I will pass through the land of Egypt this night, and will smite all the firstborn in the land of Egypt, both man and beast; and against all the gods of Egypt I will execute judgment: I am the LORD.