John Chrysostom

By this also he has quenched his anger, by delivering him up. For masters are then most enraged, when they are entreated for the absent, so that by this very act he mollified him the more. "Thou therefore receive him, that is mine own bowels." And again he has not given the bare name, but uses with it a word that might move him, which is more affectionate than son. He has said, "son," he has said, "I have begotten" him, so that it was probable he would love him much, because he begot him in his trials. …

Homily on Philemon 2 ↗

Theophylact of Ohrid

That is, receive him with love, or rather not him, but my very heart, for so much do I love him and carry him in my soul.

Commentary on Philemon ↗

Oecumenius

"whom I have sent back". To hand Onesimus over to the master, rather extinguished his anger. "to receive him as my very heart". What do I mean by "receive him"? Receiving this one, you will receive my own very "heart". Thus he says, I love him as one calls him his own "heart". And "receive" means to accept with love.

Commentary On Philemon ↗

Jerome

And what he says: "But you accept him, that is, my heart," this is what I said a little earlier, that "heart" signifies the internal affection of the heart and the full willingness from the soul, when everything that is in us is received from the one asking. But otherwise, all children are the parents' hearts.

Commentary on Philemon ↗

Thomas Aquinas

Then when he says, I am sending him back to you, he makes his request. First, he makes it, then he answers a question: I had wanted to keep him here. And so he says, and do you welcome him as though he were my very heart. And this because I have seen him changed, the sign of which is, I send him back to you. On the contrary, Deuteronomy 23:15: 'Thou shalt not deliver to his master the servant that is fled to you.' I reply that is true when the master seeks him in order to put him to death. …

Commentary on Philemon ↗