How could the Corinthians be a living epistle in a corrupt Greek culture?
They could be a living epistle by letting their conduct and practices be read by a watching world. Grace is not license, and not all things edify, so they had to be careful what they did. Even amid Greek immorality, idolatry, and mythology, they were to be something others could read and understand.
Les teaches that grace is not license, so believers cannot simply do as they please. Although all things may be lawful, not all things are expedient or edify. Every believer must be careful what they do and practice, because believers and unbelievers alike are constantly watching. Many people will never pick up this Book and read it, but they can look at you and read; you are a living epistle. Paul was trying to bring the Corinthians to understand that even though they lived in the midst of abject immorality, idolatry, and mythology, wherever they went within that Greek culture, they could be and were to be a living epistle. They were to be something the world around them could read and understand. This fits Paul’s broader case leading to the practice of the Lord’s Table.