For circumcision verily profits, if you keep the law: but if you are a breaker of the law, your circumcision is made uncircumcision.
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An opponent might say: “If circumcision is of value, why was it stopped?” It is only of value if you keep the law. Circumcision may be retained therefore, but if it is to be of any value the law must be observed. So why did Paul prohibit what he shows to be of value if the law is observed? Paul answers by saying that if the law is not kept, the Jew effectively becomes a Gentile…. But to keep the law is to believe in Christ, who was promised to Abraham. Those who are justified by faith have their own merit and are included in the honor shown to the patriarchs. For every mention of salvation in the law refers to Christ. Therefore the man who believes in Christ is the man who keeps the law. But if he does not believe then he is a transgressor of the law because he has not accepted Christ … and it is no advantage for him to be called a son of Abraham. Commentary on Paul’s Epistles.
The apostle did not say this as if he favored forcing either the Gentiles to remain uncircumcised or the Jews not to adhere to the traditions of their fathers. Rather, he urged that neither group should be forced into the practice of the other but that each person should have the right, not the obligation, to adhere to his own custom.
Circumcision profiteth indeed, inasmuch as it was ordained by Almighty God, as were also the precepts of the law, which were to be observed before the publishing of the new law of Christ. See Galatians v. 6. But it was never profitable to the transgressors of the law. Nay, the uncircumcised Gentiles, who have complied with those natural precepts, which are also commanded by the law of Moses, shall judge and condemn those, who received the written law, and at the same time were transgressors of it. (Witham)
Paul accepts the value of circumcision in theory but abolishes it in practice. For circumcision is only useful if the person circumcised keeps the law.… But a circumcised person who breaks the law is really uncircumcised, and Paul condemns him without hesitation.