John Chrysostom

"Ye were bought with a price: become not bondservants of men." This saying is addressed not to slaves only but also to free men. For it is possible for one who is a slave not to be a slave; and for one who is a freeman to be a slave. "And how can one be a slave and not a slave?" When he doeth all for God: when he feigns nothing, and doeth nothing out of eye-service towards men: that is how one that is a slave to men can be free. Or again, how doth one that is free become a slave? …

Homily on 1 Corinthians 19 ↗

Theophylact of Ohrid

He says this not to slaves alone, but also to free people, exhorting all Christians to do nothing for the sake of pleasing men and not to obey them if their commands are unlawful. This is what it means: having been bought by God, to become slaves of men. He is not urging slaves to fall away from their masters—no; this is clear from his subsequent words: "in whatever calling one was called," and so on, that is, if someone was called even in a state of slavery, let him remain in it. …

Commentary on 1 Corinthians ↗

Bonaventure

Let us not give our body to sin. "We have been bought at a great price. Do not become servants of men" nor of demons nor of sins. If I had redeemed some servant, I would not give him away for nothing.

Collationes de Septem Donis, Collation 6 ↗