What was Les alluding to when the Oklahoma man called?
He was alluding to the eternal security of the true believer — that a believer's standing before God rests on grace, not personal performance. The caller, though not angry, was upset that this teaching was being suggested at all, since it seemed to conflict with the failures and weaknesses evident in the Corinthians' lives.
Les explains that when he had only been on the Tulsa station for a month or two, a gentleman from Oklahoma called him. The man was not angry but was a little upset because Les was more or less alluding to — though not yet coming out as boldly as he later would — an eternal security for the true believer. The caller challenged him, asking how he could do that. Les ties this to the Corinthian passage: even though the Corinthians had failures and weaknesses, Paul says that if the Lord should come they would stand before Him blameless. They were not blameless in themselves, so Les asks on what basis God could do that, and answers: it's grace.