Genesis 18:14-18 meaning

Sarah's natural doubt and laughter are contrasted with God's supernatural presence. God asks rhetorically, is anything too difficult for the Lord? The birth of Isaac is set, at the appointed time God will return to you at this time next year, and Sarah will have a son.

Sarah denied it however, saying, 'I did not laugh.' The Lord's unusual knowledge startled Sarah, and she was afraid of His response. It is no wonder Sarah was afraid, she had not laughed out loud, but to herself, and her innermost thoughts had been revealed. She reverted to the natural human reflex to rationalize.

The Lord refuses to allow Sarah to get away with her lie, pointing out that she did laugh. Perhaps God was building Sarah's faith by saying, "if I know what you're thinking, don't I have the power for you to have a son?" The fact that Sarah is held up as a model of faith, yet struggled as we do, shows the mercy of God and gives us great hope. God is a God of many chances. 

Then the men rose up from there, and looked down toward Sodom. It is possible that this vantage point was the elevated village of Beni Naʿim, three miles east of Hebron. The Dead Sea, eighteen miles to the south, is visible on a clear day through gaps in the hills. Abraham was walking with them to send them off still acting as a gracious host.

God knew Abraham. He knew his character. He considered if He should tell Abraham what He was planning to do asking His two angel companions, "Shall I hide from Abraham what I am about to do?" 

The Lord decides to tell Abraham of his intention to destroy Sodom, because he promised to make him a great and mighty nation. Abraham represents the future nation of Israel, and in him all the nations of the earth will be blessed. This is the motive for God's decision not to withhold anything from Abraham (Genesis 12:2-3, 7, 13:14-17, 15:1-5, 17:1-6).