13: For the promise, that he should 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 they which are of the law be heirs, faith is made void, and the promise made of none effect: 15: Because the law worketh wrath: for where no law is, there is no transgression. 16: Therefore it is of faith, that it might be by grace; to the end the promise might be sure to all the seed; not to that only which is of the law, but to that also which is of the faith of Abraham; who is the father of us all, 17: (As it is written, I have made thee a father of many nations,) before him whom he believed, even God, who quickeneth the dead, and calleth those things which be not as though they were. 18: Who against hope believed in hope, that he might become the father of many nations; according to that which was spoken, So shall thy seed be. 19: And being not weak in faith, he considered not his own body now dead, when he was about an hundred years old, neither yet the deadness of Sara's womb: 20: He staggered not at the promise of God through unbelief; but was strong in faith, giving glory to God; 21: And being fully persuaded that, what he had promised, he was able also to perform. 22: And therefore it was imputed to him for righteousness. 23: Now it was not written for his sake alone, that it was imputed to him; 24: But for us also, to whom it shall be imputed, if we believe on him that raised up Jesus our Lord from the dead; 25: Who was delivered for our offences, and was raised again for our justification.
Paul explains that the promise that Abraham had from God, which was that he would inherit the whole earth, was not based of Abraham’s righteousness according to obedience to the law, but it was through the righteousness of faith. If those who were in the old covenant obtained the promise given to Abraham then faith was made pointless and the promise was made ineffectual. The Law brings the wrath of God on sinners and were there is no law there is no transgression of the law. Salvation is of faith so that it might then be an act of Grace on Gods part. And the blessing promised to Abraham comes not only to those who are the natural seed of Abraham but also to his seed on the basis of Faith. Abraham is called the father of many nations and he hoped in God even when it was naturally impossible to have hope. He believed that God could give life to the dead and call into being children that didn’t exist. He was not weak in faith nor did he think that his inability to bear children being about a hundred years old. Nor did he think about the inability of Sarah’s womb to bear children. He was not surprised at the promises of God but he was strong in his faith and gave glory to God. He was completely assured that God who made the promise was able to fulfill his promise and God reckoned to him righteousness. This was not just recorded for Abraham’s benefit it was written for us to know and believe too - we who believe that Jesus our Lord has been raised up from the dead. Christ was delivered for our sins and raised up for our justification.