Did Paul have to defend his apostleship in his letters?
Yes. In letters such as Galatians and Ephesians, Paul continually defended his apostleship. He asserted he was the apostle of the Gentiles and magnified his office, refusing to take a back seat to anyone, because his unique calling was not readily recognized like the Twelve's.
Les teaches that whenever he covers Paul’s other letters, whether Galatians or Ephesians or whatever, Paul is always having to defend his apostleship. This is understandable because people readily recognized the Twelve and their unique position with Christ in His earthly ministry, but Paul came on the scene like a raging bull fighting against everything that spoke of Jesus of Nazareth. In Romans 11:13, Paul writes to Gentiles, "I am the apostle of the Gentiles. I magnify my office," showing he would never take a back seat to anybody concerning his apostleship.