Why did Paul call the Corinthian believers his epistle?
Paul called the Corinthian believers his epistle because their changed lives served as a living letter. They had turned from idols and paganism to worship the one true God, and their transformation was written by the Holy Spirit on their hearts, known and read by all men.
Les explains that Paul said the Corinthians were his epistle because they were changed people. They had come out of abject paganism, immorality, and idolatrous practices and had begun to worship the one true God. Paul says, 'You are our epistle written in our hearts, known and read of all men.' Their changed lives themselves were the letter. Les says he feels the same about those who come to know the Lord through simple teaching: they become living epistles. The cited verse then calls them the Epistle of Christ, ministered by us, written not with ink but with the Holy Spirit of the living God, not on tables of stone but on believers' hearts, where the Spirit's work is written.
Ye are our epistle written in our hearts, known and read of all men: