See what happens when God promises that the son Abraham has been waiting for years for will certainly come from Sarah's womb:
"Abraham fell facedown; he laughed and said to himself, 'Will a son be born to a man a hundred years old? Will Sarah bear a child at the age of ninety?'"
The man fell over laughing! God has just promised one of the crucial pieces of the covenantal blessing, and Abraham offers an incredulous guffaw.
Sarah, likewise, is caught laughing. In the very next chapter, she is listening at the tent-flap when the visitors tell Abraham of Isaac's coming.
"So Sarah laughed to herself as she thought, 'After I am worn out and my master is old, will I now have this pleasure?'"
In both cases, God Himself responds to their laughter-- not with rejection or judgment, but with re-affirmation of his promise accompanied by even more detail: in ch. 17, Abraham is promised that Sarah will bear the child within a year! In ch. 18, they are both told that Isaac will be born within a year.
What is striking about this is how often I wrestle with incredulity. I struggle with promises from God that are far easier to believe than one that a 100-year old man will father a child with a 90+ year-old woman. What is more, the forgiveness of my sin is a finished work, accomplished because of God's steadfast promises to Abraham (in spite of his incredulity!).
I need to take to heart God's purposeful determination to accomplish redemption on behalf of His people! Ultimately, that's exactly what Abraham did:
"On that very day Abraham took his son Ishmael and all those born in his household or bought with his money, every male in his household, and circumcised them, as God told him."
Genesis 17:23 (emphasis added)
Abraham's faith was not supplanted by his incredulity; faith overcame doubt, and in belief Abraham obeyed God.