Romans 4:16-21

Therefore, [inheriting] the promise depends entirely on faith [that is, confident trust in the unseen God], in order that it may be given as an act of grace [His unmerited favor and mercy], so that the promise will be [legally] guaranteed to all the descendants [of Abraham]—not only for those [Jewish believers] who keep the Law, but also for those [Gentile believers] who share the faith of Abraham, who is the [spiritual] father of us all— (as it is written [in Scripture], “I HAVE MADE YOU A FATHER OF MANY NATIONS”) in the sight of Him in whom he believed, that is, God who gives life to the dead and calls into being that which does not exist. [Gen 17:5] In hope against hope Abraham believed that he would become a father of many nations, as he had been promised [by God]: “SO [numberless] SHALL YOUR DESCENDANTS BE.” [Gen 15:5] Without becoming weak in faith he considered his own body, now as good as dead [for producing children] since he was about a hundred years old, and [he considered] the deadness of Sarah’s womb. [Gen 17:17; 18:11] But he did not doubt or waver in unbelief concerning the promise of God, but he grew strong and empowered by faith, giving glory to God, being fully convinced that God had the power to do what He had promised.