John Calvin Commentary Philemon 1:17

"If then thou countest me a partner, receive him as myself." — Philemon 1:17 (ASV)

If, therefore, thou holdest me to be thy associate. Here he lowers himself still further, by giving up his right and his honor to a runaway, and putting him in his own place, as he will shortly afterwards offer himself to be his guarantor. He considered it to be of vast importance that Onesimus should have a mild and gentle master, so that immoderate severity might not drive him to despair. That is the object which Paul toils so earnestly to accomplish.

And his example warns us how affectionately we should aid a sinner who has given us proof of his repentance. And if it is our duty to intercede for others, in order to obtain forgiveness for those who repent, much more should we ourselves treat them with kindness and gentleness.