25Circumcision has value if you observe the law, but if you break the law, you have become as though you had not been circumcised. 26If those who are not circumcised keep the law's requirements, will they not be regarded as though they were circumcised? 27The one who is not circumcised physically and yet obeys the law will condemn you who, even though you have the written code and circumcision, are a lawbreaker.
28A man is not a Jew if he is only one outwardly, nor is circumcision merely outward and physical. 29No, a man is a Jew if he is one inwardly; and circumcision is circumcision of the heart, by the Spirit, not by the written code. Such a man's praise is not from men, but from God.
What's in Your Heart!
Paul presented his argument in favor of "What's in your heart" and not what we claim with our lips, as the factor in deciding if one is a true follower of Christ. The "circumcision of the heart" concept has been explained in many way to illustrate his point. Simply put, God desires that our heart be right with him and therefore live in obedience.