The faith of Abraham--it's an idea that inspires in Hebrews 11 and informs in Romans 4 and Galatians 3. It is the faith of Abraham, and Sarah too, that compelled them to go when God said go, to believe when the facts suggested faith was folly. Genesis 15:6 "Abram believed the Lord, and he credited it to him as righteousness." This verse is used as a part of a theological foundation to make the case that it is faith, not works or law, through which God grants his gracious salvation.
Yet the story of Abraham and Sarah is a bit more complicated than that. Yes, they showed remarkable faith. Still, it was not a flawless faith in the least. Their faith was frequently peppered with questions, riddled with doubt, and marked by fear. At times, when God seemed either unwilling or too slow to act, they took matters into their own hands to protect themselves and fulfill God's promise.
The older they got the more improbable the promise seemed. When God continued to reassure them of his plans to give them a son in order to make them into a great nation to bless all nations, they did what came naturally. They laughed. Not just Sarah, either. Both of them laughed. The reality is, without God's great intervention, the notion is laughable.
With God's miraculous gift, the notion is laughable, too. This time, though, the laughter is born, not out of despair, but out of delight. It's laughable that this couple so advanced in age would have a son. It's also laughable that God would stick with a couple with so many doubts, so many detours from God's plan. It's laughable and it is absolutely wonderful. There's hope that God can use even people given to so many doubts and detours like you and me.