This is the introduction I wrote on Philemon for my Ugandan friends:
Philemon is a very personal letter from Paul. It is a plea for the life of Onesimus, whom most scholars presume to have been a runaway slave. This letter was written while Paul was under house arrest in Rome (AD 61-63.) The situation for a runaway slave under Roman rule was dire. Punishment for the slave was often severe. Paul is sending Onesimus, the presumed runaway, back to his owner, Philemon, with this letter asking Philemon to receive Onesimus without punishment. Philemon was a man of some standing and a convert to faith in Christ under Paul’s ministry. Although Paul does not speak against the institution of slavery, this letter indicates a trajectory against slavery is developing in the growing Christian movement. Faith in Jesus Christ no longer allows us to live life as we always have lived. In Christ there is no longer slave or free, Jew or Greek, male or female.
There is not much more to say about this very short letter except this: It is difficult for modern readers and modern Christians to read in the bible an assumption that human slavery was legal, common place, and considered "normal". It is also difficult for modern readers to read the Bible and find little if any condemnation of human slavery. One of the hardest things to do is to read a document in its historical context and not read it through our own "assumed to be enlightened" view points. Paul does not openly condemn slavery -- but he does change the values and the rules around which Christians approached the issue and that change created a trajectory that gradually eliminated the legalized practice of human slavery in predominantly Christian communities. That slavery is still in existence today (legally and illegally) is a great evil and one that Christ followers should work against at every opportunity.