The basic issue in Gal. 2:15-16 is whether ἐὰν μὴ is adversative
('but') or exceptive ('except' or 'but only'). The NWT treats it as
exceptive and, admittedly, I had never stopped to verify this reading
until someone brought up the question. But I am familiar with the
construction ἐὰν μὴ (εἰ + ἄν) in Attic texts and, with those texts in
mind, I have never thought that the NWT handling of this passage was
"We who are Jews by birth, and not sinners from the nations,
recognize that a man is declared righteous, not by works of law,
but only through faith in Jesus Christ. So we have put our faith in Christ Jesus, so that we may be declared righteous by faith in Christ and not by works of law, for no one will be declared righteous by works of law."
However to make sure that my theological presuppositions are not
unduly affecting my reading of this Pauline passage, I checked a few
commentaries on Galatians and here are my findings.
Ernest De Witt Burton (A Critical and Exegetical Commentary on the
Epistle to the Galatians, pp. 120-121) writes that ἐὰν μὴ (Gal.
2:16) "is properly exceptive, not adversative." But what part of the text
does the construction except? Burton explains that it might be
the "preceding statement taken as a whole" or "the principal part of
it," but in the final analysis, he opts for ἐὰν μὴ excepting οὐ δικαιοῦται ἄνθρωπος only. He thus understands Paul to mean that we cannot be justified by works of Law at all. In order to make the English translation intelligible though, Burton suggests the reading "but only."
Ben Witherington III (Grace in Galatia: A Commentary on St Paul's
Letter to the Galatians, pp. 178-179) concurs with Burton. He
supplements Burton's observations, however, by noting that even an
adversative construal of the operative phrase would still rightly
yield the translation "but only." Witherington thinks that ἐὰν μὴ
is unequivocally exceptive, and he argues that this treatment
of the construction makes the best sense of the words in the context of Paul's letter. Even if one opts for the adversative sense here, Witherington insists, we should still understand Paul to mean "but only."
"It is hardly believable that Paul would have said to the Galatians that 'works of law' plus faith in Christ could justify. Indeed, it is precisely this sort of combination that he is arguing against in the remainder of his letter!" See page 179 of Witherington's commentary.
On the other hand, Frank J. Matera (Sacra Pagina Series) gives an interesting Catholic perspective. He seems to favor the adversative sense and he of course indicates that one can be justified by works of Law accompanied by faith in Christ. However, this reading might conflict with Paul's letter as a whole and it possibly does not do justice to Paul's well established antithesis between works of the Law and faith. So we have options for translating ἐὰν μὴ in Gal. 2:15-16, but it's also instructive how theology informs one's understanding of Greek syntax.