What did Paul want the Corinthians to understand about being a living epistle in their culture?
Paul wanted the Corinthians to understand that, though surrounded by Greek immorality, idolatry, and mythology, their lives functioned as a living epistle. Wherever they went in that culture, they were to be something the surrounding world could read and understand, so their conduct had to match grace rather than license.
Les explains that 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. Many people will never pick up the Book and read it, but they can look at a believer and read. Paul was building the case that every believer has to be careful what they do and practice, because believers and unbelievers alike are constantly watching. Grace is not license, and not all things are expedient or edify, so the Corinthians were to be something the world around them could read and understand.