John Calvin Commentary Philemon 1:13

"whom I would fain have kept with me, that in thy behalf he might minister unto me in the bonds of the gospel:" — Philemon 1:13 (ASV)

Whom I was desirous to keep beside me. This is another argument for appeasing Philemon: Paul is sending him back a slave, whose services, in other respects, he greatly needed. It would have been extremely cruel to disdain such strong affection shown by Paul. He also states indirectly that he would be gratified to have Onesimus sent back to him rather than for Onesimus to be harshly treated at home.

That he might minister to me instead of thee in the bonds of the gospel. He now mentions other circumstances:

1. Onesimus will take his master's place by performing this service.
2. Paul himself, through modesty, was unwilling to deprive Philemon of his right.
3. Philemon will receive more applause if, after his slave has been restored to him, he willingly and generously sends him back.

From this last consideration we infer that we should aid the martyrs of Christ by every kind service within our power while they are laboring for the testimony of the gospel. For if exile, imprisonment, stripes, blows, and the violent seizure of our property are believed by us to belong to the gospel (as Paul here calls them), whoever refuses to share and partake in them separates himself even from Christ.

Undoubtedly, the defense of the gospel belongs to all alike. Accordingly, the one who endures persecution for the sake of the gospel should not be regarded as a private individual, but as one who publicly represents the whole Church. Hence it follows that all believers should be united in taking care of this defense, so that they do not, as often happens, leave the gospel to be defended by only one person.