Albert Barnes Commentary Philemon 1:15

"For perhaps he was therefore parted [from thee] for a season, that thou shouldest have him for ever;" — Philemon 1:15 (ASV)

For perhaps he therefore departed for a season. Perhaps on this account, or for this reason dia touto he left you for a little time. Greek, "for an hour"—prov wran. The meaning is, that it was possible that this was permitted in the providence of God, in order that Onesimus might be brought under the influence of the gospel, and be far more serviceable to Philemon as a Christian, than he could have been in his former relation to him. What appeared to Philemon, therefore, to be a calamity, and what seemed to him to be wrong on the part of Onesimus, might have been permitted to occur in order that he might receive a higher benefit. Such things are not uncommon in human affairs.

That you should receive him for ever. That is, in the higher relation of a Christian friend and brother; that he might be united to you in eternal affection; that he might not only be with you in a far more endearing relation during the present life than he was before, but in the bonds of love in a world that shall never end.