Paul is teaching Philemon, and indeed Onesimus, to think within the biblical narrative, to see themselves as actors within the ongoing scriptural drama
I’m reading through N.T. Wright’s Paul and the Faithfulness of God; and will first post a few extracts. From page 11:
Here, as elsewhere, the short letter to Philemon provides an accurate signpost forwards to the wider Pauline concerns we shall explore throughout the present book….
…at the heart of his work is the yearning and striving for messianic unity across traditional boundaries, whether it be the unity of Jew and Gentile in the Messiah (the main point of Galatians), the unity of the church under the lordship of the Messiah in a pagan and imperial context (part of the main point of Philippians, coming to memorable expression in 2:1-4), or, as here in Philemon, the unity of master and slave, expressing again what it means to be in Christo. ‘So, if you reckon me a koinonos, a partner, prosabou auton, welcome him as you would welcome me.’ Or, as he puts it in Galatians, ‘There is no longer Jew or Greek; there is no longer slave or free; there is no “male and female”; you are all one in the Messiah, Jesus.’ That unity, as we shall argue in part II of the present book, was for Paul the central symbol of the Christian worldview.
[Starting at the bottom of page 13, there’s an important comment on Pauline allusion to the Hebrew scriptures]:
No doubt some will insist that to detect an allusion like this is out of order; that only those biblical echoes may be allowed which we can be sure Paul’s intended audience would certainly have recognized. But that is (to be frank) not how most writers write, and we may be confident that it is not how Paul thought. Take that route, and there will be nothing left remarkable under the visiting moon. Take, though, the risk of assuming that the texts’ footfalls echo in the memory of one familiar with them from boyhood; assume that there are indeed times when one can find the mind’s construction in the phrase; and the reward may be not only an insight into the way Paul’s mind worked but also a sudden clarity about what he was really saying in this particular instance. This, in verses 15 and 16, is the platform upon which Paul can then make his central appeal in verse 17. Before he gets to the question of sending Onesimus back, let alone giving him his freedom, he places the whole situation within the closest available scriptural background.
[The above in preface to ‘The Central Argument’, pp 16-22 which is too central to excerpt. Well, if I must: ‘eis Christon’]