17(as it is written, "A FATHER OF MANY NATIONS HAVE I MADE YOU") in the presence of Him whom he believed, even God, who gives life to the dead and calls into being that which does not exist. 18In hope against hope he believed, so that he might become a father of many nations according to that which had been spoken, "SO SHALL YOUR DESCENDANTS BE." 19Without becoming weak in faith he contemplated his own body, now as good as dead since he was about a hundred years old, and the deadness of Sarah's womb; 20yet, with respect to the promise of God, he did not waver in unbelief but grew strong in faith, giving glory to God, 21and being fully assured that what God had promised, He was able also to perform.22Therefore IT WAS ALSO CREDITED TO HIM AS RIGHTEOUSNESS.23Now not for his sake only was it written that it was credited to him, 24but for our sake also, to whom it will be credited, as those who believe in Him who raised Jesus our Lord from the dead, 25He who was delivered over because of our transgressions, and was raised because of our justification.
In Romans 4, Paul brought in two witnesses to prove his point that we are justified by faith alone and not by works. His main witness was Abraham. He demonstrated that God credited Abraham as righteous because he believed that God could do the impossible or make him a father of many nations. Abraham was really old and so was his wife, Sarah. There was no human way possible that they could produce a child. Abraham looked past the humanly possible and at the God of the impossible and trusted God to do what He said. It was at that point that Abraham was declared righteous.
Paul listed two things that Abraham had to believe about God. First, that He was a God who gives life to the dead as he and Sarah were way beyond childbearing years. Second, he believed that God calls into being that which does not exist. He trusted that God could make something exist that simply wasn't there before. Both of these things were supernatural events and required faith in God alone. Abraham believed and IT WAS ALSO CREDITED TO HIM AS RIGHTEOUSNESS. Paul closed this argument concerning the faith of Abraham as he explained that we are saved in the same way as this ancient father of many nations. Just as Abraham was declared righteous, not by anything he did, but by what he believed about God, so are we justified as we trust that God delivered Jesus to die and be raised Jesus from the dead for our sins.