Which issue does Paul repeatedly defend in his letters?
Paul repeatedly defends his apostleship in his letters, such as Galatians and Ephesians, because people readily recognized the Twelve's unique position with Christ and His earthly ministry but questioned his apostolic authority. He never takes a back seat to anybody concerning his apostleship, magnifying his office as the apostle of the Gentiles.
Les explains that whenever he teaches Paul's other letters, whether Galatians or Ephesians or whatever, Paul is always having to defend his apostleship. That is understandable because people readily understand the Twelve and their unique position with Christ and His earthly ministry, so they have no problem recognizing their apostleship. But Paul, who was like a raging bull fighting against everything that spoke of Jesus of Nazareth, had to magnify his office as the apostle of the Gentiles and never take a back seat to anybody concerning his apostleship.