What does 2 Corinthians 3:4 mean?

Paul has described the believers in Corinth as the only letter of recommendation he needs. Their transformation and growth is enough to establish himself as a legitimate apostle, sent by God to represent Christ to the world. Some in Corinth may have been challenging his authority in that position. Paul has pointed to the Corinthians themselves as the evidence that his ministry is genuine and established by God. After all, they have experienced God's forgiveness through faith in Christ and have witnessed the power of the Holy Spirit. All that they now believe to be true has come from Paul's delivery of the gospel to them.

Where does Paul's confidence in God's working among the Corinthians come from? How is he so sure he has truly been sent by God? It is not a confidence that comes from belief in himself. Instead, he is confident in God through Christ. When God is the one at work, confidence in what He is doing is the only rational response.