Judging by the context, Onesimus, whose name Paul mentions at the beginning of the letter, was Philemon's slave, apparently converted to Christ together with his master. Later he became Paul's companion and helper, and now, being imprisoned in a Roman prison, Paul returns Onesimus to Philemon, in his own words no longer as a slave, but as a brother in Christ.
One may think that the spiritual formation of the newly converted Onesimus took place under the apostle's direct influence; it could not have been otherwise if Onesimus really accompanied Paul for a sufficiently long time. And when his spiritual formation in Christ was completed, he ceased to be a slave, if not legally, since there is no clarity about the formal status of Onesimus and it is unclear whether Philemon freed him as a freedman or not, then in fact.
After the baptism of the master himself and all his household, Philemon's house in fact became a space of the Church, where slaves in the traditional Greco-Roman sense could not exist, and Christian slaves could not exist by definition, whatever legal status the people living there had. Thus the Kingdom entered the world, transfiguring it first of all at the level of relationships, which constitute the spiritual foundation of the Kingdom.
But to cease being a slave in fact meant to walk the path that makes a person an inhabitant of the Kingdom. That means a person inwardly absolutely free and fully responsible. A person for whom there are no longer forced or unconscious relationships with anyone. And who, accordingly, can no longer be anyone's slave, but can be a helper to anyone whom God entrusts him to help. Regardless of status, which now becomes a matter sometimes important, but secondary, determining neither the spiritual life of the former slave nor his relationships with his former masters.
