How did Paul answer people who questioned whether he was a real apostle?
Paul answered challenges to his apostleship by pointing to the Corinthian believers as proof. They had been saved out of abject paganism through his preaching, so he said, "You Corinthians are the proof of my apostleship." Their salvation was evidence enough that his apostolic ministry was genuine.
Les explains that Paul had to overcome doubts about his apostleship constantly with various statements. One of his favorites was that the Corinthians themselves were the proof of his apostleship. Because they had been saved out of abject paganism through his preaching, their salvation was proof enough that he was an apostle. Paul also would not take advantage of that responsibility by taking material reward, saying he just would not do that. He had once profited more than his equals in Judaism as a Pharisee of the Pharisees, but now he made himself a servant or bond slave of all, so that he might gain the more. His one driving desire and motivation was to see lost people saved.