Philemon’s home in Colossae was a church-meeting venue. It was from there that Onesimus, his slave, ran away and ended up meeting the then-imprisoned apostle Paul. Onesimus became a Christian, and Paul urged him to return to his master. To that end, Paul wrote this letter to Philemon.
Paul doesn’t identify himself as an apostle in the opening line, like he does in his other letters. Additionally, he doesn’t use his apostolic authority to make Philemon take back Onesimus; instead he pleads to Philemon’s love for the Lord and His people.
Even though the apostle doesn’t call for the abolition of slavery, the implications of this letter are fatal to the institution.Paul the Jewish Pharisee, Philemon the wealthy Gentile and Onesimus the lowly slave are all united by faith in Christ Jesus. Anyone who is in Christ is a brother or sister in the Lord; they are a new creation!
Sources: Alistair Begg, Dick Lucas. I also recommend John Piper’s How Paul Worked to Overcome Slavery.