This letter is in the Bible's New Testament. Paul wrote this letter to Christians in Corinth because there were false teachers in the church. They spoke badly about Paul. They said he kept changing his mind. This was because he did not visit Corinth. They also said he was not a genuine apostle*. Unlike the false teachers, Paul was not a great speaker. Also unlike them, Paul did not ask the church for money.
Paul explained why he had not visited them. He also explained why he had been so severe towards the Christians in Corinth. But he was happy that they had confessed their evil deeds to God. And now, they accepted Paul as a friend again. He was not a powerful person like the false teachers. But because he was weak, people saw God's power through him. He had not come to talk about himself. He had come to tell them about Jesus. By the death of Jesus, God had changed them from enemies into friends. Then he asked them for a generous gift for the poor Christians in Judea.
Finally he gave reasons why he was a genuine apostle*. God chose him to be an apostle*. And he had suffered many troubles because he taught God's message. But Paul was glad to work for God. This was his answer to those who spoke badly about him.
*Apostle: a person whom God sent to teach the Christian message to the world.