Romans 2:27

‘And will not the uncircumcision which is by nature, if it fulfil the law, judge you, who with the letter and circumcision are a transgressor of the law?'

Then logically if someone was naturally uncircumcised because he was not a Jew, but fully fulfilled the Law, would he not be in a position to act as judge on those who had the letter of the Law and circumcision, but were transgressors of the Law? Thus the tables would be turned. It would not be the Jew who on behalf of God judged the Gentile (which was the Jewish viewpoint), but the Gentile who on behalf of a righteous God judged the Jew, in spite of the Jew having the Law and being circumcised. Paul's whole point is that circumcision in itself does not put a person in a position of special privilege unless he ‘does what the Law says'.

It should be noted that, although he does not cite the fact here, Paul's position is supported by the Old Testament where on a number of occasions the Scriptures emphasise that it is not outward circumcision that is important, but the circumcision of the heart (which is not strictly physical circumcision). See, for example, Leviticus 26:41; Deuteronomy 10:16; Deuteronomy 30:6; Jeremiah 4:4; Jeremiah 9:26 where the command to circumcise the heart suggests that their physical circumcision is not enough for them to be truly in the covenant. What is required is a work in the heart, wrought by God.

With regard to the uncircumcised judging the circumcised compare Jesus' words in Matthew 12:41; ‘the men of Nineveh will stand up in judgment with this generation and will condemn it', for they had truly repented, unlike Israel. They were the uncircumcised who would judge the circumcised.