Circumcision, to the Jew was the outward sign of being God's chosen people. God told Abraham that the circumcision of the flesh would be a token between them for the generations to come. It reminded them of the promise God had made to Abraham.
Circumcision was a big deal to the Jews. It embodied everything they were. Paul was making no small accusation by telling them, they have become as though they had not been circumcised.
Jesus faced the same mindset. In Matthew 19, we see a rich man come to Jesus and ask him what good things he should do to inherit eternal life, and Jesus' reply was to obey the commandments. The man stated that these things he had done since he was a youth. So Jesus instructed him to sell everything he owned and follow him. The man went away from there in great distress because the Bible said that he had great possessions.
This man had followed the law since his youth and in comparison to everyone else he was pretty good. Jesus revealed to him, that even though he followed the law and had the sign of circumcision, he was still lacking in the righteousness he sought after. In the end, he chose his earthly riches over eternal life.
When the disciples had heard Jesus say that it would be easier for a camel to go through the eye of a needle than for a rich man to enter into the kingdom of God, they were amazed and utterly puzzled. They asked him who then, could be saved. Jesus said, "With men this is impossible, but with God all things are possible" ( Matthew 19:26 ). Meaning that man could never obtain salvation from observing the law, it was impossible for them. But thankfully, Jesus tells us ‘with God all things are possible'. Thank God, that he provided the plan, the way and the payment for us so that it would be possible.
Because no one could perfectly keep the law, failure to do so made the Jews, as though they were uncircumcised in the site of God. That is because the Law was not given as a way to get to God. However, Jesus was given as the way. Today, if you have never trusted in Jesus Christ as your personal Savior, I pray that you will quit relying on your goodness and put your faith in Him.