Galatians 2:18

If I build. That is, if after I, as a Jewish Christian, have recognized the utter inefficacy of the law as a means of salvation and have turned to Christ for justification, I return to the Jewish legal system in an attempt to find justification.

Things which I destroyed. That is, the ritual observances of Judaism as a means to salvation. For a Jew to accept Christ was a tacit admission that in and of itself Judaism could not save a man. Even though Paul writes in the first person, it is evident that he is thinking of what would be true of all Jewish converts to Christianity.

A transgressor. For a converted Jew to return to the practices of Judaism as a means of salvation would be to admit the inadequacy of Christ to save a man by faith alone. Furthermore, it would be an admission that he had done wrong to turn away from Judaism in the first place, and, by doing so, had made himself “a transgressor” of the ritual law. This was the very thing Peter had done (vs. 11-14), and which all Judaizers were doing.