For the promise that he would be the heir of the world was not to Abraham or to his seed through the law, but through the righteousness of faith. 14 For if those who are of the law are heirs, faith is made void and the promise made of no effect, 15 because the law brings about wrath; for where there is no law there is no transgression.
16 Therefore it is of faith that it might be according to grace, so that the promise might be sure to all the seed, not only to those who are of the law, but also to those who are of the faith of Abraham, who is the father of us all 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. 22 And therefore “it was accounted to him for righteousness.”
23 Now it was not written for his sake alone that it was imputed to him, 24 but also for us. It shall be imputed to us who believe in Him who raised up Jesus our Lord from the dead, 25 who was delivered up because of our offenses, and was raised because of our justification. — Romans 4:13-25.
Thus the promise is to all who exercise the same faith in God’s word and promise that Abraham did which made him the ‘father of many nations’. In his case it meant that, as a man of about a hundred years of age with a wife whose womb was ‘dead’, he did not waver at the promise of God but believed Him. This was counted to him as righteousness. Paul teaches that this Old Testament lesson was also written for those of us in the New Testament era as a type or shadow of the resurrection of the Lord Jesus Christ. We believe in the God of the greater miracle of raising up Christ from the dead for our justification after He had been delivered to the cross for our sins. Our faith is not simply in a crucified Saviour who bore our sin, but in a living Saviour whose resurrection was God the Father’s seal on the sufficiency of His Son’s sacrifice for us.
The promise came to Abraham because he hoped against hope! Because he believed the crazy promise that he would be a father to nations! Because he gave glory to God, fully convinced that God was able to do what was promised. [Source: The Bible Panorama and another.]