In Rom 4 and elsewhere, Paul indicates that the Jews had been radically misusing circumcision. According to Paul, circumcision pointed to the covenant righteousness that Yahweh had reckoned to Abraham on account of his faith. Paul says that circumcision was the “seal of righteousness” but Genesis 17 talks about circumcision as the “sign of the covenant,” and Paul’s transformation of the language shows that he understood “righteousness” as a covenantal term. Thus, when Isaac was circumcised his circumcision pointed to the covenant standing that he had through the faith of his father. The sign showed that he was in that covenant, a recipient of the covenant promises that had been given through Abe, that he was the beginning of a new generation of covenant partners with Yahweh. Circumcision was, as NT Wright has put it, a pointer to faith, or more precisely to righteousness through faith.
Now, Paul describes faith as the abandonment of hope in one?s self and one?s own abilities. Abraham’s faith was “hope against hope,” and a confidence in the power of God to accomplish what Abraham could not accomplish. Insofar as circumcision was a pointer to this kind of faith, it was a sign of the abandonment of hope in one?s own ability, an abandonment of boasting. Yet the Jews had turned it into an occasion for boasting, boasting, literally, in the flesh, or the absence of a bit of flesh. Paul makes a related move in Gal 4, where, without so much as mentioning circumcision, he implies that circumcision stood between Abraham’s fleshly son and the son of the promise. Cutting off the flesh was a sign that the flesh accomplishes nothing, and a sign that the flesh isn’t needed anyway, since the fulfillment of the promise depends entirely on God.