The current iteration of the NET Bible takes Romans 2:27 as a question and treats the participle τελοῦσα as adjectival. Here’s the translation:
And will not the physically uncircumcised man who keeps the law judge you who, despite the written code and circumcision, transgress the law?
As the NET editors have been wrestling with this text, we have come to some slightly, but important, differences in how we want to render this verse. This paper is a justification for what those changes will be in the second edition. As well, there is some exposition on the basic meaning of the verse below.
The Greek text has: καὶ κρινεῖ ἡ ἐκ φύσεως ἀκροβυστία τὸν νόμον τελοῦσα σὲ τὸν διὰ γράμματος καὶ περιτομῆς παραβάτην νόμου.
κρινεῖ —future tense, as is evident by the accent: “will judge.”
ἡ ἐκ φύσεως ἀκροβυστία —“the uncircumcision from nature”—that is, “the physical uncircimcision.” This is a metonymy of abstract for concrete.
τελοῦσα —This is an adverbial participle that is almost always translated as though adjectival (so NET, RSV, NRSV, ESV, NIV, TNIV, NAB, NJB, HCSB, etc.). The lack of the article with the participle, while the subject is articular, renders this adverbial: “as he keeps, by keeping, if he keeps.” (The adverbial translation is found in the KJV, ASV, NKJV, REB, and TEV.) The object is τὸν νόμον even though grammatically σὲ could also serve as object (but hardly in sense!). τελοῦσα of course relates to ἀκροβυστία since it too is feminine singular. Thus, “The physically uncircumcised man, by keeping the law, will judge you…”
σὲ τὸν —although we would expect the second τὸν to refer back to the law, this creates an awkward sense: ‘the outwardly uncircumcised man will judge you, by keeping the law, which is through the letter and circumcision.’ What is the point of describing the law—especially the law that the uncircumcised man obeys? Indeed, this is unlikely for two reasons: (1) Paul almost universally uses νόμος in an absolute way—i.e., without further qualification. There is no reason why he would begin to qualify it here when he has spoken of the law already in Romans seventeen times without qualification, all in chapter 2.1 (2) The qualification here would be particularly inappropriate since Paul is describing the law that the uncircumcised obeys.
An alternative is to see τὸν as referring back to σὲ (though the grammatical concord is only in number and case, not gender since σὲ is genderless): ‘you, who through the letter and circumcision, are a transgressor of the law.’ This creates a more satisfactory sense, though the διά needs to be massaged. However, there is no good reason why the second τὸν cannot modify the masculine accusative noun, παραβάτην: ‘you… the transgressor of the law.’) In the better Koine writers, a prepositional phrase often occupies medial position between the article and the noun.) The article, in this instance, is par excellence (within the universe of discourse of the two types of people discussed in this chapter): rather than the uncircumcised man, you—the Jew!—are the transgressor of the law! The reason for the prepositional phrase between the τὸν and the παραβάτην is to clarify that it relates to σὲ rather than to ἀκροβυστία. Further, the construction is most likely an object-complement: ‘the uncircumcised man will judge you [to be] the transgressor of the law.’2 The charge is thus stronger than is seen in normal translations.
διὰ γράμματος καὶ περιτομῆς—the preposition has the concessive force of “though provided with” here.3
Diagrammed, Rom 2:27 is as follows:
Although the NET regards v. 27 as asking a negative question (“Will not the uncircumcised man judge you…?”),4 there is no negative particle here. As the NA27 has it, this is most likely a statement: “The uncircumcised man will judge you…!”
The entire verse thus can be translated as follows: “The physically uncircumcised man, by keeping the law, will judge you to be the transgressor of the law, even though you have the letter and circumcision!”
2 With object-complement constructions the verb εἶναι or the conjunction ὡς is either implied or stated.
3 BDAG 224 s.v. διά A.3.c.
4 So NET.