For circumcision indeed is of value if you obey the law, but if you break the law, your circumcision becomes uncircumcision.  So, if a man who is uncircumcised keeps the precepts of the law, will not his uncircumcision be regarded as circumcision?  Then he who is physically uncircumcised but keeps the law will condemn you who have the written code and circumcision but break the law. (ESV)
Let's just rip off the band-aid. Circumcision is a topic that Paul will spend a lot of time on in this letter to the Romans. This act of cutting of the male foreskin was an integral part of the Jewish Law given by God. In this physical act Israelite children were set apart as belonging to God. One of the primary differences between Jew and Gentile was this physical act.
What had the Jews done with this rite of circumcision? They had insisted on the physical act while ignoring the heart of the Law. If you go back and read the history of the Jewish people you find a constant falling away from the Law of God. While they physically separated themselves in circumcision they were worshiping all of the false idols of the Gentiles around them.
Paul is making clear that circumcision only means anything if it is accompanied by obedience. You cannot claim to be a circumcised follower of God while ignoring God's commands. The one who gave the command for circumcision also gave the command for loving your neighbor, for not having other gods, for speaking truthfully with your neighbor, etc.
Paul gives one powerful jab to the Jews who would have been reading this letter. He says the uncircumcised Gentiles who are obeying the law are truly circumcised and will condemn the Jews who claim circumcision only in the physical sense. That statement would have been a scathing indictment to Jews in the 1st Century.
We are not bound any longer by circumcision in a physical sense. Jesus fulfilled the Law. However, consider your claim to know God. Do you obey his words? Does your atheist neighbor live a more godly life than you? Words to ponder.