Paul has been so adamantly defending the Gospel message in his letter to the Galatians, and stressing that it is by faith that the Spirit moves, not works. Why is Paul so adamant? This is hugely significant because Paul was once a Pharisaic Scholar, meaning that he was not only an advocate for the Law of Moses, but He was also a zealous defender of it. What changed him was that he met Jesus face to face, it cost him his sight, and the Holy Spirit moved him to such a degree, both in theology and experience, that he ends up writing scripture (and in that writing most of the New Testament). Paul challenges the reader to question what sense does it make to start with faith and from there, perfect yourself when you can't change you? You have to yield yourself to Christ and from there, allow Him to do the changing from within, and the Holy Spirit is instrumental in accomplishing this.