John Gill

And shall not uncircumcision which is by nature,.... That is, the Gentiles, who are by nature uncircumcised; for as circumcision was by the command of God, and performed by the art of men, uncircumcision is by nature, and what men naturally have. Now if it, such persons, fulfil the law in Christ, they will judge thee, the circumcision: and condemn, as Noah condemned the old world, Heb 11:7, and the men of Nineveh and the queen of the south will condemn the men of that generation, in which Christ lived, Mat 12:41. Who by the letter and circumcision transgress the law; that is, either by the law, which is "the letter", and "by circumcision", or "by circumcision which is in the letter", Rom 2:29, sin being increased by the prohibitions of the moral law, and the rituals of the ceremonial law, and the more so by a dependence upon an obedience to either of them, or both, for justification.