Why did Paul constantly have to defend that he was God's man?

Les explains that the Corinthians were still carnal, not a spiritual church like Ephesus or Philippi, but a church beset with all kinds of problems, beginning with divisions. Some said they followed Paul, some Apollos, some Peter, and some even claimed to follow Christ. As Paul addressed those divisions, he had to defend his apostleship. Les points out that everywhere in Paul's letters he is defending his apostleship. In his early ministry Paul always went first to the Jew, to the synagogue, where he might get a halfway civil hearing; when the Jews rejected it, he went out into the Gentile area, the marketplace, and wherever else. But wherever he went there was that constant slur that he was an imposter, not a true representative of Judaism, a renegade Jew. So he always had to come back and defend the fact that he was God's man for the hour. Even though the Corinthians were carnal, they were still co-laborers with the Apostle, and that puts us in the same place, as co-laborers together to bring to fruition the eternal will of God.