Unlike other letters that he dictated, Paul personally wrote this letter to Philemon. (Verse 19) The apostle was then a prisoner, and a subscription in a number of manuscripts identifies the location to have been Rome.
Paul did not explain how it happened that he crossed paths with Onesimus, but simply related that he had become a father to him during the time of confinement. As a result of the message about Christ that the apostle shared with Onesimus, he became a believer, and a deep bond of affection developed, comparable to that of a father and his beloved child. (Verse 10)
Although Paul would have greatly appreciated continuing to benefit from the help Onesimus was able to provide, he sent him back to Philemon, his master. As a slave in the household of Philemon, Onesimus had formerly been “useless.” That, however, had changed on account of his becoming a believer, one who sincerely desired to honor God and Christ and thus was willing to serve. Therefore, Paul confidently referred to Onesimus as having become useful to him and to Philemon. (Verse 11) In this letter, besides commending Philemon highly for his love for fellow believers and his faith, Paul kindly requested that he welcome Onesimus back as more than a slave, as a “beloved brother.” (Verses 16, 17)
In his letter to the Colossians (4:7-9), Paul mentioned that Tychicus would be coming with Onesimus. This indicates that Philemon’s home was in Colossae, and his house there served as a meeting place for the community of believers. (Verse 2)
It may be noted that Paul did not attack the then-existing social system that allowed slavery. The manner in which he spoke about Onesimus, however, reveals a complete acceptance of him as a deeply loved equal partner in God’s family of children. In disposition, the apostle adhered to the truth he had enunciated in his letter to the Galatians (3:28, 29) that, in Christ, there is no more slave or free. This unqualified acceptance of others as having an equal standing before God transcends what the mere abolition of the demeaning system of slavery can accomplish. History amply demonstrates that the abolition of slavery does not end hateful prejudice and false pride related to race, national or tribal origin, or social standing. As in the case of Paul, though, the operation of God’s spirit on believers does break down divisive barriers and produce the marvelous changes for good that even noble human efforts simply cannot.
See http://bibleplaces.com/colossae.htm for pictures of and comments on Colossae.