How did the Holy Spirit help the Corinthians understand the gospel?
The Holy Spirit helped the Corinthians understand the gospel by enabling them to grasp that they were sinners for whom Christ died and rose from the dead. Their understanding came not through human eloquence, philosophy, or smooth talk, but through the power of God working when the simple gospel of Christ was preached.
Les explains that Corinth was near Athens, so the Corinthians were surrounded by philosophy and paganism and were used to hearing philosophers with smooth talk and big words. Paul refused to come with that kind of language, choosing instead to come with power. Les teaches that it was not Paul's fast talking or language that brought these people out of paganism, but the power of God when he preached the simple gospel of Christ. The Holy Spirit then helped those people understand that they were sinners for whom Christ died and rose from the dead. This matches the point that the kingdom of God is not in word but in power.