Which is by nature - Which is the natural state of man; his condition before he is admitted to any of the unique 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 Matthew 12:41-42.
Who by the letter ... - 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 pagan 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 Luke 16:6-7; Acts 28:21; Galatians 6:11, particularly the writings of Moses, denoting, by way of eminence, the letter, or the writing; Rom 7:6; 2 Timothy 3:15.