Ambrosiaster — on Phlm 1:10 (Commentary on Philemon)

“It is not a great thing to humble oneself when one is lowly, but it is commendable for a noble man to humble himself. Therefore, the Apostle asks him, to whom he has the authority to command, to be in all things a model for the advancement of merits. Therefore, when Onesimus, a fugitive, returned to seek divine assistance, the Apostle baptized him while he was in custody in the city of Rome, seeing the hope of usefulness in him.”