Why could Paul say he was an apostle to the Corinthians even if others did not recognize him?

Les explains that Paul's answer was the Corinthians themselves. Paul could say that if he was not an apostle to others, yet doubtless he was to them, because he had seen them saved out of abject idolatry only because of his ministry. They were the proof of the pudding; if he were not an apostle, they would still be in their paganism. Les applies the same test to his own critics: people may ask who he thinks he is, a layman without education, but his proof is many people saved out of abject sinful backgrounds and brought out of darkness into the light of the Gospel. That is what Paul is saying—his apostleship was proven by the Corinthians themselves, because he had led them out of dark idolatry. The complaint against Paul, especially from the Jewish community, was that he was not one of the Twelve and had not walked with Jesus three years like Peter, but the fruit of his ministry among the Corinthians answered that.