For this perhaps is why he was parted from you for a while, that you might have him back forever, no longer as a bondservant but more than a bondservant, as a beloved brother—especially to me, but how much more to you, both in the flesh and in the Lord. Philemon 1:15-16 [ESV]
I really like the thought Paul has here in 15 & 16. Essentially, he is saying that maybe Onesimus needed to run away that he might find Christ for himself…and in this then return to Philemon forever as a brother in Christ rather than his bondservant. Paul knows this because Onesimus has been that (a brother) to him.
When Paul says “both in the flesh and the Lord” he means in a practical and helpful way as well as a spiritual way. Practical and helpful now because before he ran away, he had stolen from Philemon and betrayed his trust. Now as a ‘New Man’ Onesimus will be so much more to Philemon than he was. On top of that Onesimus will be a spiritual member of the body providing in those ways as well; being used by God for the furthering of Philemon’s church.
This may seem simplistic but when it is applied is very powerful. Understanding the potential impact of a born-again human being is not something to be underestimated. When the Lord grabs ahold of a wayward heart, the profound transformation we often see is staggering; it grows our faith!