He's telling the Romans that they don't have to become Jews to become Christians. The earliest Christians were Jews, of course, including, it is worth repeating, Jesus Himself, or himself--depending on your particulars. And in keeping with his people's custom, he was circumcised--a thing that Paul is telling the Romans they needn't be. This is good news on any given day, at least to the men of the congregation. It is a concept they can get behind. (Then, as now, women were left out of important portions of the conversation.) For early Christians, the old laws about diet and fashions and the keeping of feasts were easy enough and all in line with the rules of good living. But circumcision--the deal that was cut between Abraham and God--required a commitment, and for the non-Jew, Paul knew, it was likely a deal breaker.
So he's trying to tell the Romans it's not all that important after all. He's floating this option of "spiritual circumcision." It's a talking point, and the numbers look good. It's not what you do, he's telling them, but that you believe that makes the difference. It's not what happens to your body. It's what happens to your soul. It is not "works" but "faith."
Then too, he doesn't want to offend the brethren back in the Promised Land, who are his kinsmen and the Chosen People and, by the way, all circumcised, to the man. If he devalues the old covenant of blood made between God and Abraham--the Old Testament--and their attention to "the law," he's going to lose the very ones who have made the leap of faith about the Nazarene--the part about Him being the Son of God--and who form the Jewish core of the movement. Try telling some co-religionist who just had his foreskin removed that it really wasn't necessary--that what they did had no special meaning, that a good Jew's foreskin isn't worth a gentile's faith. These are the issues that breed wars and Reformations. And 15 centuries later, Martin Luther, tired of the hoops the church had its people jumping through, quotes Paul to the effect that "faith alone" will save us.
Are we saved by what we do--circumcision? By what we don't do--drink, dance, eat meat on Fridays? Or by what we have faith in--Christ's divinity?