Theophylact of Ohrid

He says this excellently, writing with spiritual grace and as if saying: for greater assurance that I will pay the debt, my own handwritten letter serves as guarantee. Lest it seem reproachful to Philemon that the apostle, as though not trusting him, does not simply ask him concerning the slave-thief, but also gives a written guarantee in his own hand — for this would signal that Philemon is hard-hearted and inattentive to his teacher — the apostle (by the words cited) gives him to understand that he trusts him, and greatly so. …

Commentary on Philemon ↗

Jerome

I, Paul, have written this with my own hand. I will repay it, not to mention that you owe me your very self." What he says is this: "Whatever Onesimus may have wronged you, I will repay. I, Paul, write this with my own hand. I will not remind you that you owe me your very self. But I say this as speaking to a stranger. Moreover, if I should regain my right due to the message of Christ, which I have preached to you, and you have become a Christian, you owe yourself to me. …

Commentary on Philemon ↗

Thomas Aquinas

Therefore he says, I, Paul, as if to say, that you might be certain of restitution. I write it with my own hand. And this not out of necessity, because you owe me your very self, because I snatched you from eternal death, and thus he should do this for his liberator. Tobias 9:2: 'If I should give myself to be your servant, I should not make a worthy return for your care.'

Commentary on Philemon ↗