Of all the books of the New Testament, Philemon is the one I understand least. I mean, I get it: it's a letter by Paul, and so it's in the Bible. But why? At first glance, this is just a letter to a slave owner asking him not to punish the slave Paul has had with him. Paul never even explicitly asks Philemon to free Onesimus (it's implied, but never actually stated), and that fact was exploited by slave owners for many, many years. So why include this letter? Why Philemon?
There are a few things we can say about Onesimus himself that may point us in the right direction. For starters, he was useless to Philemon, "but now he has become useful both to you and to me" (v. 11). We are all useless to God and the kingdom of heaven until we accept Christ. Slaves to sin, we can do nothing good whatsoever; as a matter of fact, we are worse than useless -- we actively work against Him! When we are saved, though, we can work for the Lord and be useful. Next, look at Paul's progression in the way he wants Philemon to think of Onesimus: "no longer as a slave, but better than a slave, as a dear brother [ . . . ] a fellow man and as a brother in the Lord" (v. 16). As we come free of our chains of sin, we are no longer its slaves. We are human beings, family in God, co-heirs with Christ. Our Christian journey is one from bondage into freedom.
Again, Paul never demands Onesimus' freedom in so many words; he asks for it out of love. That's our model for how to be Christians in this world. We may be weak, we may hold positions of power, but we are to always act out of love. Grace, not force, wins the day.
Take another look at Paul's letter from prison to Philemon. Thank God for freeing you from your slavery.