Colossians 3:11 says, “Here there is not Greek and Jew, circumcised and uncircumcised, barbarian, Scythian, slave, free; but Christ is all, and in all.” And it is set in a pericope that speaks of the Christian’s life of putting off the old man and putting on the new. Christ has created, via the church, a new humanity that is a restoration of fallen humanity in the Garden. Note the various divisions: religious, ethnic, socio-economic, that are healed in Christ. Pay particular attention to slave vs. free.
Colossians is written after the book of Philemon and the two cross over each other. In Philemon in verses 1 and 2 we find that Philemon, a slave owner, has a church in his home which is apparently in Colossae. In the letter Paul urges Philemon to set Onesimus, a slave, free. Colossians is likely written several years later and in it we find that this very Pauline letter is being brought to the Colossian church by a certain likely former slave. Colossians 4:9 reads “and with him Onesimus, our faithful and beloved brother, who is one of you.”
So there is very distinct possibility that Onesimus, the former slave is sitting in the Colossian church that meets in the home of the former slave owner, Philemon. They may be sitting side by side as the letter, with it’s declaration of the new humanity, is read. That’s how radical the kingdom of God is.