Verse 17: But if, while we seek to be justified by Christ, we ourselves also are found sinners, is therefore Christ the minister of sin? God forbid. The we in this verse is a reference to believing Jews. His point to Peter here is that he would be a sinner if he tried to get the Gentiles to do things under the law that are no longer required under grace. God forbid he says!
Verse 18: For if I build again the things which I destroyed, I make myself a transgressor. In other words, to put himself or others back under the law would make him a sinner and a false prophet. Again, the point here is that by bringing people who have been saved by grace back under the law would be in essence be rebuilding what Paul calls the middle wall of patrician in Ephesians 2:11-18 that was removed by. Make no mistake, the Devil is trying, and successful for the most part, at getting the church to put this wall back up. Every time we do it, the wall gets higher, e.g., Don't smoke, Don't chew, Don't date girls who do. One brick at time!
Verse 19: For I through the law am dead to the law, that I might live unto God. What does he mean through the law am dead to the law? His point is that through grace he is now dead to the law, or, it could be that he is saying that the law killed him, but grace gave him life again. He wrote in 2 Timothy 1:10, But is now made manifest by the appearing of our Saviour Jesus Christ, who hath abolished death, and hath brought life and immortality to light through the gospel:
Verse 20: I am crucified with Christ: nevertheless I live; yet not I, but Christ liveth in me: and the life which I now live in the flesh I live by the faith of the Son of God, who loved me, and gave himself for me. Just as Christ died because of the law, so did Paul, and now his life is the result of the faith of the Son that sent him to the cross in his stead. He is not saying that Christ's faith saved him, but that it provided the means of his salvation. In other words, our faith in his completed work on Calvary is what saves (cf. verse 16) and Christ's faith made our salvation possible. Other translations try to take this message away for some reason. Just look at how the NASB, NIV, NKJV, etc. translates it, and verse 16 for that matter. Personally, that is why I just stay with he KJV.
Verse 21: I do not frustrate the grace of God: for if righteousness come by the law, then Christ is dead in vain. What he means by not frustrating the grace of God would be to leave grace and return to the law. That frustrates grace! To frustrate means to put in place of, i.e., to put law in the place of grace.
Furthermore, he says that righteousness does not come by the law, at least not in this dispensation.
Just to clarify what Paul is saying here in verses 15-21, he is speaking to believing Jews and it will not make sense and will contradict other Scriptures to see it otherwise (verse 15). Some of these believing Jews wanted to place themselves back under the law (verses 18-19), but Paul tells them that is not possible for those who have been freed by grace to go back under the law because they are dead to it now in and through Christ (verse 20). Finally, he warns them to stop frustrating the grace of God in their attempt to do so (verse 21).