I thought this section on Wright’s Justification was very helpful. Page 119
Galatians 2:17-18 raise and answer a question which must, like Galatians 2:16, be solidly anchored to the actual situation Paul is describing in 2:11-14.
“If, while seeking to be justified in the Messiah, we ourselves turn out to be ‘sinners,’ does that make the Messiah a servant of sin? Certainly not!” In other words (addressing Peter), “Yes, we are seeking to find our identity as God’s people `in the Messiah,’ trusting in his ‘faithfulness’; and yes, that means that in terms of the Torah as we know it we find ourselves standing alongside ‘Gentile sinners,’ as in 2:15. Technically, we are ‘sinners’ like them.” This, I think, is preferable to the obvious alternative, which is to understand this as a reference to something like Luther’s simul iustus et peccator: we are justified in Christ, but still sinners simply in the sense of committing actual sin.
But this does not mean—as some, perhaps those who had come from James, might have inferred—that the Messiah was simply stirring up “sinful” behavior, encouraging people to kick over the traces and live “outside the law” along with . . . those Gentile idolaters! Certainly not! Rather, “if I build up again the things which I tore down, I demonstrate myself to be a transgressor.” Paul has moved from “we” to “I” at this point, preparing for the intensely personal, and deliberately rhetorical, appeal of Galatians 2:19-21. What he is saying can be spelled out like this: “If, having pulled down the wall of partition between myself and the Gentiles, having discovered that it is abolished through the Messiah, I then build it up again by separating myself from the Gentiles, all I accomplish is to erect a sign (the Torah itself!) which says ‘you have transgressed.’” “Transgression,” we should note, is the actual breaking of the law, whereas “sin” is any missing-of-the-mark, any failure to live as a genuine human being, whether or not the law is there to point it out.