What doctrine was Les alluding to when the Oklahoma caller called?
The doctrine being alluded to was the eternal security of the true believer. The context explains that the Corinthians, despite failures, would stand blameless before God if Christ returned, and that this standing rests on grace rather than personal merit.
Les explains that the Oklahoma caller was upset because Les had been alluding to eternal security for the true believer, though he had not yet stated it as boldly as later. Les connects that security to 1 Corinthians 1:8, where Paul says Christ will confirm believers unto the end so they may be blameless, and asks on what basis God could do that. The answer is grace: the Corinthians were not blameless in themselves, yet Paul says they would stand blameless before Him if the Lord came.