For no one is a Jew who is merely one outwardly, nor is circumcision outward and physical. (ESV)
I remember being very excited on Halloween night as a child. We never celebrated anything that was dark or evil. We celebrated chocolate! As a young boy I was able to put on a mask and a costume and for one evening I was someone else. I remember being Fred Flintstone one year. I may have looked like him, but no one really thought a cartoon character had come to life. It was just a costume.
It is one thing for a small child to use their imagination and a clever mask and think that they are something that they are not. It is quite another for an adult to put on an outward display and somehow think that they are something that they are not. Paul is making the case here that this is essentially what many of the Jews were doing.
The Jews very much liked to "look the part." From a physical standpoint they were circumcised. They would also be seen at the feasts. Many of them would make a point to be seen going to the synagogue to pray. Spirituality for them was in many respects an act. If anyone was observing them from a physical and outward perspective they would be impressed with them as a Jew.
Paul says, if the essence of you being a Jew is simply what others observe outwardly you are missing the point. It is almost as silly as me thinking I was Fred Flintstone because I had a mask and a costume. Yelling "yaba daba doo" did not make me the cartoon character. What is on the inside is what matters.
Are you a believer in Jesus Christ? Is your heart truly his? Or are you simply going through the motions so that others think you are a Christian. Take off the mask. Give your heart to Jesus.