What’s happening here is Paul is addressing the matter of a false preacher in their midst who had been saying that Paul was not who or what he said he was. And this false preacher was trying to bring false teaching into the church.
Remember, the church was still very young. This was only a few years after Jesus walked the earth. Satan, the enemy of man’s souls, was trying to undermine the church through whatever means he could. Since Paul was the most effective apostle there was, the enemy really wanted him stopped.
So Paul, in this part of his second letter to the Corinthians, is defending himself, not for his own good, but for the good of the church. And not just the Corinthian church, but for “the” church. And he also pointed out that the gospel this false teacher brought them was not the Gospel that Paul was teaching, and the Jesus the false teacher was preaching was not the Jesus who met Paul on the Damascus road. He was not Jesus of Nazareth, son of the living God.
And friends, there are still false teachers today, bringing false messages of false gospels and false Jesuses. It is important to discern the true from the false. You need to know what the Bible says and you need to know the Jesus of the Bible. Don’t be fooled by cheap imitations. They are all around. Some are easy to spot, but some are cleverly disguised. The Bible says to compare what you hear them say with what you read in its pages. If they disagree with the written Word of God, then throw them out.