Why is Paul always defending his apostleship in his letters?

Paul defended his apostleship because people readily recognized the Twelve's unique position with Christ and His earthly ministry, yet Paul had formerly been like a raging bull opposing everything about Jesus of Nazareth. As apostle of the Gentiles, he magnified his office and refused to take a back seat to anyone.

Les explains that in Romans 11:13 Paul writes to the Gentiles as the apostle of the Gentiles, magnifying his office. Les notes that Paul never takes a back seat to anybody concerning his apostleship. When teaching Paul's other letters, such as Galatians or Ephesians, Les always comments that Paul is having to defend his apostleship. This is understandable because people readily understood the Twelve and their unique position with Christ and His earthly ministry, so they recognized their apostleship without difficulty. But Paul came as one who had been like a raging bull fighting against everything that spoke of Jesus of Nazareth. That background helps explain why his apostleship was continually defended.

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.