John Gill Commentary Romans 2:27

"and shall not the uncircumcision which is by nature, if it fulfil the law, judge thee, who with the letter and circumcision art a transgressor of the law?" — Romans 2:27 (ASV)

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, (Hebrews 11:7) , and the men of Nineveh and the queen of the south will condemn the men of that generation, in which Christ lived, (Matthew 12:41Matthew 12:42) (Luke 11:31Luke 11:32) .

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", (Romans 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.