Why were the Corinthian believers proof that Paul was an apostle?

Les explains that Paul constantly had to overcome opposition and defend his apostleship, and one of his favorite statements was that the Corinthians themselves were the proof of his apostleship. They had been saved out of abject paganism as a result of his preaching, so their salvation was proof enough that he was an apostle. Paul then showed that he would not take advantage of that tremendous responsibility by seeking material reward, though he had done so when he was in Judaism.