2 You are our epistle written in our hearts, known and read by all men;
The answer to the second question in verse 1 is found here in verse 2. The Corinthian church’s transformation by Christ was all the recommendation that Paul’s team needed.
From a human standpoint, Paul was the pen that wrote the living letter of the church in Corinth. He was the human instrument, Christ was the Author of their salvation, and the Holy Spirit was the means of winning the congregation to Christ.
2 You are our epistle [letter] written in our hearts,
Paul’s team (Paul, Timothy, Titus) needed no letter of recommendation from Corinth because they brought the gospel to the Corinthians in the first place. Their very existence as a church manifestly demonstrated the validity of the team. Their testimony was their credentials. Previously they were pagans; now they were Christians, a divine transformation having occurred.
The phrase “written in our hearts” is language derived from the New Covenant (Jer 31:31-34). It was the gospel team’s heart for sharing the gospel with the Corinthian church that has emphasis here. Paul’s team carried the Corinthians in their souls wherever they went.
known and read by all men;
The existence of the Corinthian church served as a letter of recommendation for Paul’s team. He was their spiritual father (1 Co 4:15; 9:2). Anyone in Corinth could see what happened to them. Everyone could know and read about the transformation that occurred in the hearts of the Corinthian church just by observing their testimonies.
Christians are the primary Bible that non-Christians read.
The only letter this world possesses whereby they can see the reality of the gospel is the living letters of Christians. Non-Christians comprehend more than what we say; they want to make sure our testimony is consistent. When they come to grips with a genuine testimony, they have a clear opportunity to be exposed to authentic Christianity. God employs us toward that end. The consistent Christian life is a powerful testimony to the world. Every Christian is an open letter from Christ to the world.
What message do unbelievers read from our lives? Do they see the reality of Christ in how we live? The world watches Christians to see if what they have is real or not. Do they observe anything that would make our Lord wonderful to them?