This chapter is targeted toward the Jews in the church of Rome. As he has already done in Galatians, Paul shows that Abraham was made right with God based on his faith, not the things he did. In Galatians, Paul pointed out that Abraham was considered righteous before the law even existed. Here he points out that it was even before he was circumcised. Neither the law nor circumcision has the power to make a person righteous. If salvation is by faith, it cannot have anything added to it or else it is the result of the thing the person has done. Salvation becomes a form of payment for meeting a requirement. The only requirement is to believe Jesus died for our wrongdoing.
Mixing faith and something we do doesn’t work. If there is anything required of us, we could then claim to have played a part in our salvation. God has determined that he wants to do all the work and give us salvation as a gift. Think how insulting it would be for you to give someone a gift that has cost you dearly and the receiver offers you a quarter for it. That gives a small insight into how God feels when we attempt to earn his gift of salvation.