The separation of these classes was vital to the Roman economy since more than a third of the population is comprised of slaves. But this gulf that separated master an d slave has been bridged through Christ. There is now neither slave nor free man, for all believers are one in Christ Jesus, and all are accorded the same standing and privileges before God. If this was Paul's intention, he would have based his appeal on the common ground that Philemon and Onesimus shared as humans created in the image of God. But in Onesimus case, Paul takes the issue further and bases his stand on the spiritual brotherhood that has enjoined both Philemon and Onesimus together.
Spiritual blood is thicker than physical blood.
Some observations can be made by looking at how the physical family functions. First of all, I come from a family of six children and I didn't have one bit of say in deciding who my siblings should be. In th e same manner, Philemon did not have a say as to whether Onesimus should be allowed to become his brother. Also, I'm called to love all men, but I have a greater responsibility to care for my family members. The same principle is also applied in the spiritual family.