Why did Paul constantly have to defend his apostleship to the Corinthians?
Paul had to defend his apostleship because the Corinthian congregation was divided. Some claimed they followed Peter or Apollos, and some dismissed Paul as a fake or imposter. They argued Peter had authority because he walked with Jesus for three years, while Paul had not, so Paul continually reaffirmed his apostolic calling.
Les explains that the whole letter of Corinthians deals with a series of problems in that little congregation. Beyond moral and relational issues, they were divided over human leaders: some said they listened only to Peter, others to Apollos. Some even told Paul he was a fake and an imposter, insisting Peter had the authority because he walked with Jesus for three years, while Paul did not. Therefore Paul had to constantly defend his apostleship, reminding them that he was judged by no man except Jesus Christ, the one who sent him out as the apostle of the Gentiles.