At this point, Paul himself is a Roman prisoner "for Christ Jesus." From a worldly perspective, like Onesimus, whom he affectionately calls "my own heart," the apostle's freedom is greatly circumscribed. In this set of circumstances it would seem that only Philemon is "free." Paul sends Onesimus back because he "belongs" to Philemon even as he pleads with him to welcome back his runaway slave as he would welcome the apostle himself, as "more than a slave," as "a brother," gently reminding him that all of them, Paul included, belong to Christ.
It is interesting, though easy to exaggerate, I suppose, that the apostle writes that Philemon will have Onesimus back "forever." The Greek word translated as "forever" is aionios, which means, in this context, "without end, never to cease, everlasting." It is a word that appears no less than 67 times throughout the New Testament, usually in passages not as seemingly mundane as this one. I believe that such a thought could only emerge from the apostle's pen in light of both Philemon and Onesimus being brothers in the Lord and his spiritual discernment that Onesimus going back was somehow important either to both of their salvation, or to Philemon's, who may have been embittered by his slave departing to be with the apostle.
Jesus tells us in our Gospel for this Twenty-third Sunday in Ordinary Time that we if are His we are not our own, that following Him does not just cost us something, but everything, even our very life. The Lord calls us today to live with detachment towards earthly goods, choosing Him before all else. St. Paul exercised the detachment called for by Jesus when he sent Onesimus, whom he loved dearly, back to Philemon for the Lord's sake, which means for Philemon's and likely Onesimus' sake, the sake of their salvation, which, at least it seems to me, Paul understood needed to be worked out together in some way.
Just as in last week's Gospel, we see once again how difficult it is not to put ourselves, our own needs, concerns, and wants before the good of others. To do so is exactly what it means not only to carry own cross, but to embrace it, relying on the Lord, trusting in His mercy, goodness, and tender care. Indeed, the Lord is our refuge. Besides, there is no person, apart from the Lord, who is capable of shouldering the weight of our great need. Comprehending this by experiencing it for yourself, verifying it in reality through the circumstances you face, is truly liberating.