What does 2 Corinthians 10:9 mean?

Paul has been defending himself from a charge by some in Corinth. False teachers, people bitter about being called out for sin, and competing leaders have claimed Paul is an unimpressive speaker in person, though he is bold and strong in his letters to them.

The ability to speak with eloquence and good arguments was highly valued in Greek culture. Believers in Corinth would have grown up hearing some of the best traveling speakers in their society, making presentations on all kinds of philosophical topics. Paul, apparently, was much less polished and impressive as a speaker, when compared with the secular presenters of the day. He does not argue against the charge in the following verse that "his bodily presence is weak, and his speech of no account."

Instead he says here that the point of his "bold" letters is not to frighten his readers. Most likely, what Paul implies is that he is not bluffing: these are not empty threats. His words are warnings he will follow through when he is with them in person. They should believe he will back up his written words when he shows up among them. Paul will go on to say that the impressiveness of his speaking is less important than what he actually does when he is with them.