(Formerly he was useless to you, but now he is indeed useful to you and to me.)
τόν ποτέ σοι ἄχρηστον νυνὶ δὲ [καὶ] σοὶ καὶ ἐμοὶ εὔχρηστον,
Ver. 11. Which in time past was to you unprofitable.
See how great is his prudence, how he confesses the man's faults, and thereby extinguishes his anger. I know, he says, that he was unprofitable.
But now he will be profitable to you and to me.
He has not said he will be useful to you, lest he should contradict it, but he has introduced his own person, that his hopes may seem worthy of credit, But now, he says, profitable to you and to me. For if he was profitable to Paul, who required so great strictness, much more would he be so to his master.
[For moral, see Phm 1:16]
Haydock Catholic Bible Commentary: Philemon
Who heretofore was unprofitable to thee, in taking and spending what belonged to thee, yet now, after a sincere conversion, is profitable both to me and thee; to me, by the services he has done me in prison; and the joy I have had by his conversion; and also to thee, because I know thou wouldst have been glad to have rendered me all possible services thyself, and he has done them for thee; he hath supplied thy place. For these reasons I could have wished to have detained him with me: but I have sent him back, thou being his master, nor would I do any thing in regard of thy servant, without thy advice and consent, that if thou thinkest it fitting to send him back again to me, and to give him his freedom, it may be without any constraint upon thee, without any necessity, thy voluntary and charitable act and deed. Wi. — S. Paul here makes an allusion to the word Onesimus, signifying useful in the Greek. He was before unprofitable, he says, to thee, contrary to the import of his name; but now he is truly an Onesimus, or useful, both to you and to me; to you indeed, by his conversion, and the resolution he now makes to serve you faithfully the remainder of his life; to me also, by the services he renders me in my chains. Calmet. — S. Jerom observes that some hypercritics pretended that this subject was not deserving the solicitude of an apostle, and on that account questioned its author; but this reasoning is unworthy of those who adore a God who did not refuse to die for rebellious and impious slaves. It shews pastors how solicitous they should always be for the salvation of the meanest of their flock; yes, though they may appear obdurate, and dead and buried in the pit of sin.