Rom 4:16 So the promise is received by faith. It is given as a free gift. And we are all certain to receive it, whether or not we live according to the law of Moses, if we have faith like Abraham's. For Abraham is the father of all who believe.
The promise to Abraham remains valid. Abraham did not have a Bible, Moses was not born yet and he believed because God asked him to. If someone asked you to believe in them as God of everything and you offspring would produce many nation and you are 99 years old as is your wife. Well let’s just say in this day and age there may be a few doubters. Abraham had the key ingredient, complete faith in God. The promise to Abraham can be found here Gen 17:1-2
Rom 4:17 That is what the Scriptures mean when God told him, "I have made you the father of many nations." This happened because Abraham believed in the God who brings the dead back to life and who creates new things out of nothing. Rom 4:18 Even when there was no reason for hope, Abraham kept hoping—believing that he would become the father of many nations. For God had said to him, "That's how many descendants you will have!" Rom 4:19 And Abraham's faith did not weaken, even though, at about 100 years of age, he figured his body was as good as dead—and so was Sarah's womb. Rom 4:20 Abraham never wavered in believing God's promise. In fact, his faith grew stronger, and in this he brought glory to God.
Life Application New Testament Commentary: In spite of all this, Abraham was clearly human and imperfect. He had his weaknesses (fears) and bad habits (lying under pressure). The Bible describes Abraham with all his flaws, but as a man of faith. Thus, Abraham’s faith could not have been anything but simple trust in God. Yet God honored that. Abraham is a model not just for the Jews, but for all people as a person of faith who realized he was totally dependent on his Creator for all things, even life itself.
Abraham is now a centenarian. In modern day America we have a few of those, they generally are in homes with wheel chairs etc. I am sure none of them expect to have a child.
Rom 4:21 He was fully convinced that God is able to do whatever He promises. Rom 4:22 And because of Abraham's faith, God counted him as righteous. Rom 4:23 And when God counted him as righteous, it wasn't just for Abraham's benefit. It was recorded Rom 4:24 for our benefit, too, assuring us that God will also count us as righteous if we believe in Him, the One who raised Jesus our Lord from the dead. Rom 4:25 He was handed over to die because of our sins, and He was raised to life to make us right with God.
In these last 10 verses Paul maps out what we have to do to be accepted by God. It is so very simple. But saying it will do nothing for you unless you believe in Christ’s under all circumstances. There are many reasons to doubt that is what makes faith so tough. If you look and seek God will continue to show you the way. Abraham believe and he was “counted as Righteous”.