What does Genesis 18:15 mean?
The Lord and Sarah have been having a conversation without looking at each other. The Lord has been speaking to Abraham, fully aware that Sarah, behind him inside of a tent, is hearing every word. In this conversation, He has told Abraham for the second time, Sarah for the first, that she will have a son by this time next year. When Sarah laughed at the idea, the Lord revealed He knew she laughed to herself and He even knew her thoughts. He revealed that He was the Lord, that nothing was too hard for Him, and that she would have a son.Now, oddly, Sarah chooses to deny that she had laughed. This silly lie feels beside the point when the Lord has told her that she will, miraculously, have a baby in a year's time. The Lord repeats that she did laugh. And yet, we're told Sarah was afraid. Perhaps she thought the Lord may punish her in some way for her unbelieving laughter. Or, for listening in on a conversation in which she was not explicitly involved.
He does not condemn her, though. In fact, as God revealed to Abraham in the previous chapter when Abraham also laughed at such an idea, their son's joyful name would be Isaac, which means "laughter." This name not only evokes happiness, it also serves as a subtle, almost playful rebuke of Abraham and Sarah's mutual doubt.