Through his teaching, Paul has “destroyed” (2:18) “the barrier of the dividing wall” between Jew and Gentile which is “the Law of commandments contained in ordinances” (Eph. 2:14-15).
“Through faith in Christ” consists of two concepts:
(1) “Through faith,” which is faith in God and
(2) “In Christ” which indicates a mysterious unity between the believer and Christ, who is the Vehicle through which God rescues people. See the discussion in the article, In Christ.
2:17 But if, while seeking to be justified in Christ, we ourselves have also been found sinners, is Christ then a minister of sin? May it never be! 2:18 For if I rebuild what I have once destroyed, I prove myself to be a transgressor.
Galatians 2:16 indicates that man is “justified by faith in Christ”, but 2:17 omits the words “by faith,” simply saying that people are “justified in Christ”. This important issue is discussed in more detail in the page “In Christ”. The conclusion on that page is that Christ is the Vehicle through which we are saved. He “gave Himself for our sins so that He might rescue us from this present evil age, according to the will of our God and Father” (1:4). Without Christ, there would not have been any rescue.
When Paul writes that “we ourselves have also been found sinners” (2:17) he is not saying that “we” commit sin, but rather, as explained by 2:15, that “we” do not live according to the Jewish laws and traditions (2:14).
The question “is Christ then a minister of sin?” (2:17) seems to be an argument used by Paul’s opposition. Paul taught that Gentiles should not be circumcised and should not “live like Jews” (2:14). It seems as if Paul’s enemies argued that this teaching makes of Christ is “a minister of sin”.
That which Paul “once destroyed” (2:18) probably refers to something that he demolished by his teaching. In the context, where Paul resist the efforts of Christian Jews from Jerusalem to compel Gentiles to be circumcised and to live like Jews, what Paul “once destroyed” may refer to “the barrier of the dividing wall” between Jew and Gentile which, according to Paul’s teaching, Christ “broke down” (Ephesians 2:14). This “dividing wall” is “the Law of commandments contained in ordinances” (Eph. 2:15). By his teaching, Paul demolished this Law. To “rebuild” the “dividing wall” would be a transgression of God’s will.