John Chrysostom
"Worthy," he says, "of every honor." Do not think that because you are faithful, you are also free. This freedom consists in being a slave still more. If an unbeliever sees that through faith people fall into self-conceit, he will often revile it as a teaching that disposes people to rebellion. But if he notices that they are obedient, he will submit all the sooner and will be more attentive to what is said. Thus, if the faithful are not obedient, both God and the preaching will be reviled. But what, you ask, if the masters are unbelievers? Even in that case one must submit for the sake of the name of God.
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On verses 1-2. Paul commands and enjoins the same thing in his letters to the Ephesians and Colossians (cf. Eph 6:5 and Col 3:22). From this it is clear that he rejects not external slavery, but the slavery that comes from vices. This occurs even among the free and is the heaviest slavery, even if the person subject to it is free.