In Philemon 19 it says: “I, Paul, write with my hand: I will repay…” Paul is telling Philemon that if his runaway slave Onesimus owes him (Philemon) anything, he (Paul) will for sure repay whatever it is to help restore the relationship between the two. It’s interesting that Paul notes he’s writing at this point in the letter, in the body and not the conclusion, like he did elsewhere. And he doesn’t say “I wrote this *letter*.” It could be he is referring to the following phrase: “I will repay!” It would then go like this: I write with my hand: I will repay!!!” The NLT puts verse 19 in ALL CAPS to show that perhaps Paul literally wrote that part of the letter as a legal signature that he would for sure repay what he promised: I PAUL, WRITE THIS WITH MY OWN HAND: I WILL REPAY IT….” Perhaps Paul’s secretary wrote the other parts of the letter, but Paul wanted his signature here as a guarantee to Philemon and the house church there that he’d do what he said.
While I certainly disagree with aspects of J. D. G Dunn’s theology, I like his comments on Philemon 19:
ἐγὼ Παῦλος ἔγραψα τῇ ἐμῇ χειρί, ἐγὼ ἀποτίσω· ἵνα μὴ λέγω σοι ὅτι καὶ σεαυτόν μοι προσοφείλεις. In an unusual step Paul evidently took the stylus in his own hand at this point and both signed his name (“Here is my signature: Paul,” NEB/REB) and wrote out his personal guarantee (“Here, I will write this with my own hand: I, Paul, will pay you back,” GNB). It would be necessary to state what he was doing since the letter was not purely personal (where change of penmanship would be sufficient visual indication of the author’s personal intervention; see Weima 46–47) but was for public reading.
The step was unusual for Paul, since elsewhere his personal autograph marks the beginning of the letter’s closing (see the introduction to the comments on vv. 8–20). But here it comes as the climax to Paul’s appeal to Philemon, where he is pulling out all the stops and putting the full weight of his personal standing behind his words (cf. the “I, Paul” of 2 Cor. 10:1; Gal. 5:2; 1 Thes. 2:18). In this case the personal autograph does not have the function of legitimating the letter as Paul’s (see on Col. 4:18), but rather has a legal function as Paul’s personal guarantee to Philemon on behalf of Onesimus. The legal character of the procedure is put beyond doubt by Paul’s use of ἀποτίνω, which occurs only here in the New Testament, but, once again, is common in the papyri as a legal technical term meaning “make compensation, pay the damages” (BAGD, MM). Paul was not content to make promises and provide mere reassurances; rather, he undertakes the formal legal responsibility to make good whatever wrong Onesimus has done Philemon.
James D. G. Dunn, The Epistles to the Colossians and to Philemon: A Commentary on the Greek Text, New International Greek Testament Commentary (Grand Rapids, MI; Carlisle: William B. Eerdmans Publishing; Paternoster Press, 1996), 339–340.
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