Heidegger's Bible Handbook: Philemon: Occasion of the Epistle
2. The Occasion: the fugitive Onesimus falling into his net. The Argument: his reconciliation.
The occasion for writing was furnished by Onesimus, a fugitive slave, who, fleeing to Rome with a conscience trouble by theft, falls into the net of the Apostle, although a captive himself. Therefore, Paul presses him, now converted to Christ, to reconcile with his master, Philemon. Which, since it appears difficult to him, with all the craft of eloquence he approaches his master, angry with him for just causes, and perhaps being of a harsher temperament according to the character of the Phrygians; he begs pardon for the contracted guilt, and interposes his own credit for all loss.