Belief is a powerful thing. Here in this passage we are shown just how powerful.
Abraham and Sarah are old. I don't mean Tom Leary old (30) or even umm, never mind, (I don't want to get myself in trouble by mentioning how old other people are). Regardless, they are far beyond child rearing age or even being able to get around with energy, yet God’s plan was for them to bear and raise a child. Personally I have a hard time keeping up with Rachel at times with my minor aches. They not only had a child but also had the energy to successfully raise that child.
They believed even when it seemed impossible for God's promise to be fulfilled. They did stumble in that belief, but who doesn't? They were only human, just like us. In the end God makes the impossible happen.
I have had an amazing life. It's been terrible at times for sure, but every moment has led to who I am and the incredible gift God has made this life. Even though I have hard times, I do continue to believe that God has His plan and I just need to follow it. While I do believe, does my belief equal Abraham's? I will continue to pray and model my faith with his.