"Having confidence in thy obedience, I write unto thee." What stone would not these things have softened? What wild beast would not these things have rendered mild, and prepared to receive him heartily? After having borne witness to him by so many great testimonies of his goodness, he is not ashamed again to excuse himself. He says, Not barely requesting it, nor as commanding it, nor arbitrarily, but "having confidence in thy obedience I wrote unto thee." What he had said at the beginning, "having confidence," that he also says here in the sealing up of his letter.
"Knowing that thou wilt also do more than I say." At the same time in saying this he excited him. For he would have been ashamed, though for nothing else, if having such credit with him as this, that he would do more than he said,-he should not do so much.
"Confident in your obedience, I have written to you, knowing that you will do even more than I say." Whoever presumes of him whom he is about to ask, prejudices himself in a certain manner, that it may not be lawful for him to deny what he is asked. Moreover, if he who asks knows that he will be asked for more than he has asked, he seeks smaller things, that he may have the voluntary compliance of the person asked, and a greater reward in return. But if Philemon does these things according to the precept of man: how much more will he do them for the love of God? Therefore, he is justly praised by the voice of the Apostle, because he anticipated his commands by his actions and could say, "Let the voluntary offerings of my mouth be pleasing to you, Lord" (Ps. 119:108), doing more than the precept required, he conquers those who only did what was commanded and are ordered to say, "We are useless servants because we have done what we were obliged to do" (Lk. 17:10). Virginity is also awarded a greater prize for this reason: because the precept of the Lord does not have it and it extends beyond what was ordered.
"confident". Paul said, "You do this not for me, but for Christ."
"confident in your obedience". I did not write to you as one commanding or stubbornly dealing with you, but having confidence "in your obedience". Whom would these things not soften, even if he were made of iron?
"knowing that you will also do more than I say". Paul said this, persuading Philemon in every way to accept Onesimus, if not for any other reason, then at least so as not to destroy his own reputation if he had any with the Apostle.
Then when he says Trusting in your compliance, he provides a reason on the side of Philemon, and a commendation of his obedience. First he shows how he is confident in his obeying; second he adds to it something similar. Therefore he says, Trusting in your compliance. 2 Corinthians 7:16: 'I rejoice that in all things I can have confidence in you.' 1 Samuel 15:22: 'For obedience is better than sacrifices.' But he writes more cautiously because a man listens more closely to one he expects to see again than if he despairs.
[AD 384] Ambrosiaster on Philemon 1:21