(27) And shall not uncircumcision which is by nature, if it fulfil the law, judge thee, who by the letter and circumcision dost transgress the law? (28) For he is not a Jew, which is one outwardly; neither is that circumcision, which is outward in the flesh: (29) But he is a Jew, which is one inwardly; and circumcision is that of the heart, in the spirit, and not in the letter; whose praise is not of men, but of God.
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The law he is writing about here is obviously the Ten Commandments. Within this context is the Bible's definition of what God means by circumcision. Circumcision is broadly defined as "when one keeps the law." Uncircumcision is "when one breaks the law." He does not mean an occasional breaking of the law but consistently breaking it as a practice or as a way of life.
It was the shocking disparity between what the Pharisees urged others to do and what they did themselves that ignited Jesus' strong rebukes against them. Here, Paul accuses the typical Jew—not necessarily the Pharisee, the scribe, or the Sadducee—of bringing blasphemy against God by doing the same thing the Pharisees did. They taught and demanded one thing of others and did something else.
The Jews, then, had acquired a bad reputation throughout the Roman Empire by teaching one thing and doing another in the business of life. Thus, Paul says that, spiritually, they were uncircumcised. The average Jew was externally in conformity with the Covenant, but inwardly, as shown by the way that he lived his life—how he conducted his business, his family life—he may just as well have been as uncircumcised as a Gentile! There is a powerful lesson in this for us.
— John W. Ritenbaugh