The Law and the Gospel are the two things humankind need in order to live well and die well.
One of the key concerns in the Book of Galatians is that the Judaizers had mishandled the interpretation of the OT and this is of tremendous significance for the Apostle Paul.
When you are united to Christ, Christ lives in you and produces the holiness called for by the Law.
In the Book of Galatians, Paul's life unavoidably intersects with the doctrine he espouses.
Before moving on to address correct doctrine, the Apostle Paul's own apostolic ministry is detailed as an essential part of his defense of the Gospel.