What does Genesis 18:18 mean?

In the previous verse, the Lord, two angels, and Abraham stood looking at Sodom in the distance. God, apparently speaking to the angels, asks a rhetorical question. His statement is not a request for information; it's meant to point out God's purpose in this encounter. Should God hide from Abraham what He was about to do with Sodom? The expected answer, and the meaning of the comment, is that God has no intention of hiding His plans from Abraham.

Here, God continues to explain to the angels why He will not hide from Abraham what He is about to do. Abraham, the Lord says, will become a great and mighty nation. More, all of the nations of the earth will be blessed in Abraham. In other words, God has already given His astounding covenant promises to Abraham, along with His plans for Abraham's future offspring. Why would God now withhold from Abraham information about His plans for Sodom?

This also means that God's actions against Sodom are meant to carry a message to men like Abraham. What eventually occurs in that city will not only prove that God always makes good on His promises, it will also prove that God's judgment on sin is unmistakable.