'VE heard it said that Abraham was not a Jew. Well, I can see the point. Abraham (who is called "the father of the faithful" and who held a very prominent place in paradise) was not a circumcised man when God accepted him. Above that, not one word of Moses' law had even been written yet. So, the Jews of Paul's day (who had such great reverence for Abraham) really did have things turned upside down. Or, we might say that they had their head buried in the sand. Even if a person tried to extrapolate from Abraham's good works (e.g., leaving Ur, trying to have a son, offering Isaac, etc.) that there was a principle of justification by works by which he was living, we couldn't back it up with the Old Testament record of Abraham's life. Genesis 15:6 teaches plainly that Abraham was justified by faith (a truth that we will certainly revisit repeatedly throughout the epistles). This chapter hits this issue from many angles, but the lesson is always the same: faith (not effort) is the way into God's favor. As much as our fallen nature drives us to think that we can earn our worth by diligence, it still isn't true.
Now, Paul was not teaching that circumcision was meaningless. In fact, Romans 4:11 describes Abraham's circumcision as a sign and of his righteousness. He was already a believer before his circumcision. He was already in. But his circumcision represented the reality (kind of like baptism does for believers in our dispensation). If circumcision (or any other good work) were the vehicle of salvation, then faith wouldn't be necessary. But, if God's promise of grace is the only thing that is needed, then faith is infinitely powerful. He can save to the uttermost (Hebrews 7:25). After all, God's power and knowledge are infinite (Romans 4:17 & 21). He raised His Son from the dead physically, so He certainly will have no difficulty in raising our dead spirit back to life.