V.3 The Corinthian assembly or church was going on so badly that Paul says that it mattered little to him what their opinion of him was. And he adds that he could not even trust his own opinions of himself.
V.4 Means that he didn’t know of anything in his life that was wrong. But he adds that this was not the deciding thing. It was the Lord — the Master of his life — who decided. But he did not have a bad conscience. Do we?
V.14-21 The Corinthians were very critical of Paul, and while he lets this go on, he warns them of the power he has from God, as God’s Apostle.