Why does Paul keep defending his apostleship in his letters?
Paul defended his apostleship because God specifically designated him to carry salvation to the Gentiles, yet contemporaries accused him of heresy and being an imposter since he had not walked with Jesus during those three years as Peter, James, and John had. He therefore repeatedly affirmed that he truly was an apostle who received divine revelations.
Les explains that Paul was the man God designated to take salvation to the Gentiles, so he made no apology for emphasizing Paul’s authority. Because even people in Paul’s day accused him of heresy and of being an imposter—since he had not actually walked with the Lord Jesus those three years like Peter, James, and John—Paul had to come back constantly and insist, “But I AM an Apostle. I AM the one who has received all these revelations.” Les also notes that Paul did not set himself up as Savior or as some God; rather, as Paul followed the resurrected, ascended Lord, he told believers to follow him.