Why was Paul always defending his apostleship in his letters?
Paul defended his apostleship because Judaizers constantly attacked him, claiming he was an imposter and a turncoat against Jewish tradition, and that he lacked authority because he had not walked with Christ for three years like Peter and the Twelve. He responded that his apostleship came from God, not men.
Les teaches that Paul was always defending his apostleship because he was under constant attack, primarily by the Judaizers. They accused him of being an imposter, a turncoat who had turned his back on the traditions of the Fathers. They also argued that he did not have the authority Peter had, because he had not walked with Christ for three years as Peter and the twelve had. Paul therefore kept coming back to the declaration that he was an apostle, not declared such by any group of men, but by God Himself.