John Calvin Commentary Philemon 1:10

"I beseech thee for my child, whom I have begotten in my bonds, Onesimus," — Philemon 1:10 (ASV)

I beseech thee for my son. Since less weight is commonly attached to those prayers which are not founded in some cause of just commendation, Paul shows that Onesimus is so closely related to him as to provide a good reason for petitioning on his behalf. Here it is important to consider how deep is his condescension, when he gives the name of “son” to a slave, a runaway, and a thief.

When he says that Onesimus has been begotten by him, this must be understood to mean that it was done by his ministry, and not by his power. To renew a human soul and form it anew to the image of God—is not a human work, and it is of this spiritual regeneration that he now speaks.

Yet because the soul is regenerated by faith, and faith is by hearing (Romans 10:17), on that account he who administers the doctrine holds the place of a parent. Moreover, because the word of God preached by man is the seed of eternal life, we need not wonder that he from whose mouth we receive that seed is called a father. Yet, at the same time, we must believe that, while the ministry of a man is efficacious in regenerating the soul, yet, strictly speaking, God himself regenerates by the power of his Spirit. These modes of expression, therefore, do not imply any opposition between God and man, but only show what God does by means of men. When he says that he had begotten him in his bonds, this circumstance adds weight to the commendation.