“For circumcision indeed is of value if you obey the law, but if you break the law, your circumcision becomes uncircumcision.” -Romans 2:25
The Jews gloried in circumcision and regarded it as being most important. They believed it unthinkable that anyone who possessed this physical sign could lose his salvation. But Paul dashes this erroneous concept onto the rocks of reality.
Paul knew that circumcision was the sign of admission to the Covenant (Gen. 17:9–14), but it was not, itself, the Covenant (i.e., 1 Cor. 7:19; Gal. 5:3). The Covenant was an inward reality marked by a physical one. The sign (circumcision) is the seal of the thing it signifies (covenant redemption). Circumcision, therefore, was a sign of one’s commitment to wholeheartedly keep the Law of God. Without perfect obedience to the law, circumcision would be nothing more than empty pretense.
John Calvin states, “it always happens, that those who dare to set up their own merits against the righteousness of God, glory more in outward observances than in real goodness; for no one, who is seriously touched and moved by the fear of God, will ever dare to raise up his eyes to heaven, since the more he strives after true righteousness, the clearer he sees how far he is from it. But as to the Pharisees, who were satisfied with imitating holiness by an outward disguise, it is no wonder that they so easily deluded themselves.”
Baptism is much the same for Christians today. It is of no avail unless one lives as a member of the covenant people of God who trust in and follow Christ.