17 (as it is written, "I have made you a father of many nations") in the presence of Him whom he believed--God, who gives life to the dead and calls those things which do not exist as though they did; 18 who, contrary to hope, in hope believed, so that he became the father of many nations, according to what was spoken, "So shall your descendants be." 19 And not being weak in faith, he did not consider his own body, already dead (since he was about a hundred years old), and the deadness of Sarah's womb. 20 He did not waver at the promise of God through unbelief, but was strengthened in faith, giving glory to God, 21 and being fully convinced that what He had promised He was also able to perform (Romans 4:17–21).
I find Abraham’s story wonderfully optimistic! Way beyond the years of child-bearing, he believed Sarah in her dotage would conceive a child. In today’s world, women rarely give birth to a child after the age of 40. Given that they live to be around 80, child-bearing happens in the first half of their lives. Sarah lived to be in her 120s. Half of her life would have been in the 60s, yet she gave birth to Isaac when she was 90. They believed God. They were happy with His promise. They probably danced around in their tents and laughed a lot.
That’s how faith acts. It speaks about the coming event as if it has already happened. Though they cannot see, since the thing does not yet exist, they talk about it as if it does exist, and they can clearly see it with their imaginations. Faith is the exact opposite of practical reasoning, and it does not waver. Faith does not swing back and forth from belief to unbelief and back again. If God said it, He is fully capable of performing what He has said, and He, Himself, dances with you when you believe Him for impossible dreams!