Quote from Barth Vader:
I shudder to bring to a devout christians' attention [or former devout christian], an oversight regarding the meaning of Scripture.
I will assume, given your self proclamation, that you are familiar with the Mosaic standards for servitude contained not only in the passage you quote, but also in Leviticus xxv and Exodus xxi. The passage of Deuteronomy is, as you know, pertaining to foreign slaves/fugitives who set foot on Hebrew soil. I mention this because Paul fulfills this requirement with regards to Onesimus.
A reading of the epistle will make several items rather clear.
1) Onesimus obviously was a fugitive from his master's household. It could be, given Philemon was a resident near Colossae, a journey of approximately 1,000 miles to Rome, and as Paul has alluded too in his letter, that some money might have been stolen by Onesimus to aid in his journey.
2) Paul obviously gave Onesimus shelter in his own maintained household.
3) Paul did not betray Onesimus to the Roman authorities, as the secular law would have instructed. Paul did not hold Onesimus until Philemon could travel to Rome and retreive his "property".
4) Paul instructed Onesimus in the good news of Christ. Onesimus became a follower of Christ.
5) Paul respected the rights of Philemon, as allowed for by the secular laws of the time, and knowing that the Philemon was also a household of faith, sent [ by the willing acceptance of Onesimus himself] Onesimus back to Philemon with the letter of introduction we now call the Epistle of Paul to Philemon.
6) Paul knew that Philemon would recall that he himself has a Master also.
We are all slaves and servants of our lusts and desires, and sadly, even in this day and age, there are still slaves in the secular sense.
As a former devout christian, I would have thought that you would have known that the chains of bondage are broken in the heart, not on top of the flesh.