Albert Barnes on Romans 2:27

Which is by nature. Which is the natural state of man; his condition before he is admitted to any of the peculiar rites of the Jewish religion.

If it fulfil the law. If they who are uncircumcised keep the law.

Judge thee. Condemn thee as guilty. As we say, the conduct of such a man condemns us. He acts so much more consistently and uprightly than we do, that we see our guilt. For a similar mode of expression, see Mt 12:41,42.

Who by the letter, etc. The translation here is certainly not happily expressed. It is difficult to ascertain its meaning. The evident meaning of the original is, "Shall not a heathen man who has none of your external privileges, if he keeps the law, condemn you who are Jews; who, although you have the letter and circumcision, are nevertheless transgressors of the law?"

The letter. The word letter properly means the mark or character from which syllables and words are formed. It is also used in the sense of writing of any kind, (Lk 16:6,7, Acts 28:21, Gal 6:11) particularly the writings of Moses, denoting, by way of eminence, the letter, or the writing, Rom 7:6, 2Ti 3:16.

(h) "fulfil the law" Mt 12:41,42