“You are our letter, written in our hearts, known and read by all men” (2 Corinthians 3:2)
The kind of letter Paul is describing is a letter of reference or recommendation. It’s a reminder that every believer is to be a letter of recommendation for Christ. Long before someone reads a Bible in the pursuit of Truth, they will read you and me. Like it or not people are watching and making judgments regarding the genuineness of the faith we profess.