Paul wrote this letter on behalf of Onesimus, who may have been a runaway slave who had wronged his owner Philemon. We don't know what Onesimus had done, but perhaps he ran away after stealing money. Paul says in verse 18 that if Onesimus owes anything, Philemon should charge it to Paul's account. It is thought that Philemon lived in Colossae; in the letter to the Colossians, Onesimus (the slave who fled from Philemon) and Archippus (whom Paul greets in the letter to Philemon) are said to be members of the church there.