Romans 4

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.  v.13

Read chapter 4
The promise to Abraham by God that he would be a father of many nations was 430 years before the law as given.  Here where it says "heir of the world" doesn't mean he's going to be the king of the world, but that he's going inherit a portion in the kingdom, the coming world.  Because of his faith in God's promise it was accounted to him as righteousness, having nothing to do with upholding the rules and commands of the law that was later given to the people Israel (Abraham being their forefather).  These believing Jews that Paul is writing to in this letter to Romans were still so hung up on religion than faith in Jesus Christ.  Paul wasn't annulling the law at this time, but he was telling Jewish believers that it's not by upholding the law that they are saved or else faith is null (v.14). The law was to reveal their sin for without the law there is no transgression (v.15).  So faith is according to grace so that the promise is available to all and not just those that can uphold the law (v.16).  This promise of Abraham was inherited to all the Jews if they had faith because he was their father.  Paul was showing this example of Abrahams faith that it is not by works that please God but faith.  
Having faith as a gentile believer doesn't make you a son or daughter of Abraham, but it does make you a child of God!  Think of it today, you may of many godly father figures in your life but just because they are an image of a father to you doesn't mean you become their biological family, but what you learned from them has helped shaped and grown your faith.
Who are some godly father figures in your life that have helped you in your faith walk with God?