Paul’s letter to Philemon about his slave Philemon is unique. While it is definitely written to the church (he does mention other people and a house church), he is primarily talking to one person about a slave who has become a Christian, and whom Paul wants to be free to continue as a believer instead of a slave. This is a pretty hefty appeal, not only to release Onesimus from penalties, but to treat him as a brother in Christ. It’s a challenge to Philemon’s faith, and hopefully Philemon responded positively.