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Imagine a group of people who are teaching against Jesus Christ and his salvation with Grace by Faith. Galatia is taken over by a popular group of people who communicate a foreign gospel - one that really is no news about God and Christ at all (no gospel v 7

The problem Paul faces however is that people are completely critical of his communicating the truth about Jesus Christ. They are wanting to see their own interperetation that they want to see involving a work of circumcision instead of saving faith in Christ which the Bible shows us has never changed in history; only for him to communicate this is bad for the Heretics business. They want to make their money and popularity off the people who have believed in their case for circumcision over saving faith in God.
Paul must therefore give his case not just for the truth, but also for his legitamacy. Without it he can't really make a credible case to the people he wishes to see saved as the heretical teachers will produce lies and discredit Paul as a means for their "keeping a good case" with their victims.

Paul's case is laid out in verse 1 with his statement "not through the agency of men" and by his statement: :For am I seeking the favor of God or of men? Or am I trying to please men? If I were still trying to please men I would not be a bondservant of Christ (1:10)