And the Lord said to Abraham, “Why did Sarah laugh, saying, ‘Shall I surely bear a child, since I am old?’
Genesis 18:13 (NKJV)
Perhaps God asks Abraham why Sarah laughed because she shouldn’t have been startled at this point in the story. She should have laughed when Abraham first told her about it back in chapter 17 when God said Sarah would have a son.
This means that Abe never told her.
The rebuke or teaching opportunity is for Abraham, not Sarah. He is the one with something to learn.
And perhaps the lesson is here in verse 17.
And the Lord said, “Shall I hide from Abraham what I am doing?”
Genesis 18:17 (NKJV)
In the previous chapter, the text introduced the two-way Covenant and a new relationship of oneness with God, where God shares both His glory and his heartache with Abraham. So shouldn’t that be how Abraham views his wife, too? Shouldn’t she be privy to his hopes, dreams and fears?
Perhaps we are seeing the last moment of lack of faith on Abraham’s part regarding the promised son: he believes God, but not enough to tell his own wife about it.
So God gently nudges him here in chapter 18. With Sarah’s laughter.